diff --git a/doc/source/ref-changelog.md b/doc/source/ref-changelog.md
index 05ad1a64f1a2..d06f889fe4fb 100644
--- a/doc/source/ref-changelog.md
+++ b/doc/source/ref-changelog.md
@@ -12,9 +12,9 @@
 
   The types of the return values in the docstrings in two methods (`aggregate_fit` and `aggregate_evaluate`) now match the hint types in the code.
 
-- **Unify client API** ([#2303](https://github.com/adap/flower/pull/2303))
+- **Unify client API** ([#2303](https://github.com/adap/flower/pull/2303), [#2390](https://github.com/adap/flower/pull/2390), [#2493](https://github.com/adap/flower/pull/2493))
 
-  Using the `client_fn`, Flower clients can interchangeably run as standalone processes (i.e. via `start_client`) or in simulation (i.e. via `start_simulation`) without requiring changes to how the client class is defined and instantiated.
+  Using the `client_fn`, Flower clients can interchangeably run as standalone processes (i.e. via `start_client`) or in simulation (i.e. via `start_simulation`) without requiring changes to how the client class is defined and instantiated. Calling `start_numpy_client` is now deprecated.
 
 - **Update Flower Baselines**
 
@@ -28,7 +28,7 @@
 
 - **General updates to the simulation engine** ([#2331](https://github.com/adap/flower/pull/2331), [#2447](https://github.com/adap/flower/pull/2447), [#2448](https://github.com/adap/flower/pull/2448))
 
-- **General improvements** ([#2309](https://github.com/adap/flower/pull/2309), [#2310](https://github.com/adap/flower/pull/2310), [2313](https://github.com/adap/flower/pull/2313), [#2316](https://github.com/adap/flower/pull/2316), [2317](https://github.com/adap/flower/pull/2317),[#2349](https://github.com/adap/flower/pull/2349), [#2360](https://github.com/adap/flower/pull/2360), [#2402](https://github.com/adap/flower/pull/2402), [#2446](https://github.com/adap/flower/pull/2446), [#2493](https://github.com/adap/flower/pull/2493))
+- **General improvements** ([#2309](https://github.com/adap/flower/pull/2309), [#2310](https://github.com/adap/flower/pull/2310), [2313](https://github.com/adap/flower/pull/2313), [#2316](https://github.com/adap/flower/pull/2316), [2317](https://github.com/adap/flower/pull/2317),[#2349](https://github.com/adap/flower/pull/2349), [#2360](https://github.com/adap/flower/pull/2360), [#2402](https://github.com/adap/flower/pull/2402), [#2446](https://github.com/adap/flower/pull/2446))
 
   Flower received many improvements under the hood, too many to list here.
 
diff --git a/src/py/flwr/client/__init__.py b/src/py/flwr/client/__init__.py
index 4ec1082190f2..56bfadc558c3 100644
--- a/src/py/flwr/client/__init__.py
+++ b/src/py/flwr/client/__init__.py
@@ -19,18 +19,14 @@
 from .app import start_numpy_client as start_numpy_client
 from .client import Client as Client
 from .numpy_client import NumPyClient as NumPyClient
-from .numpy_client_wrapper import to_client as to_client
 from .run import run_client as run_client
 from .typing import ClientFn as ClientFn
-from .typing import ClientLike as ClientLike
 
 __all__ = [
     "Client",
     "ClientFn",
-    "ClientLike",
     "NumPyClient",
     "run_client",
     "start_client",
     "start_numpy_client",
-    "to_client",
 ]
diff --git a/src/py/flwr/client/app.py b/src/py/flwr/client/app.py
index 880e2e80dbef..a74568b8e418 100644
--- a/src/py/flwr/client/app.py
+++ b/src/py/flwr/client/app.py
@@ -17,11 +17,12 @@
 
 import sys
 import time
+import warnings
 from logging import INFO
-from typing import Callable, Optional, Union
+from typing import Optional, Union
 
 from flwr.client.client import Client
-from flwr.client.typing import ClientFn, ClientLike
+from flwr.client.typing import ClientFn
 from flwr.common import GRPC_MAX_MESSAGE_LENGTH, EventType, event
 from flwr.common.address import parse_address
 from flwr.common.constant import (
@@ -40,7 +41,7 @@
 
 
 def _check_actionable_client(
-    client: Optional[ClientLike], client_fn: Optional[ClientFn]
+    client: Optional[Client], client_fn: Optional[ClientFn]
 ) -> None:
     if client_fn is None and client is None:
         raise Exception("Both `client_fn` and `client` are `None`, but one is required")
@@ -57,7 +58,7 @@ def start_client(
     *,
     server_address: str,
     client_fn: Optional[ClientFn] = None,
-    client: Optional[ClientLike] = None,
+    client: Optional[Client] = None,
     grpc_max_message_length: int = GRPC_MAX_MESSAGE_LENGTH,
     root_certificates: Optional[Union[bytes, str]] = None,
     transport: Optional[str] = None,
@@ -124,7 +125,7 @@ class `flwr.client.Client` (default: None)
         # Wrap `Client` instance in `client_fn`
         def single_client_factory(
             cid: str,  # pylint: disable=unused-argument
-        ) -> ClientLike:
+        ) -> Client:
             if client is None:  # Added this to keep mypy happy
                 raise Exception(
                     "Both `client_fn` and `client` are `None`, but one is required"
@@ -209,8 +210,7 @@ def single_client_factory(
 def start_numpy_client(
     *,
     server_address: str,
-    client_fn: Optional[Callable[[str], NumPyClient]] = None,
-    client: Optional[NumPyClient] = None,
+    client: NumPyClient,
     grpc_max_message_length: int = GRPC_MAX_MESSAGE_LENGTH,
     root_certificates: Optional[bytes] = None,
     transport: Optional[str] = None,
@@ -223,9 +223,7 @@ def start_numpy_client(
         The IPv4 or IPv6 address of the server. If the Flower server runs on
         the same machine on port 8080, then `server_address` would be
         `"[::]:8080"`.
-    client_fn : Optional[Callable[[str], NumPyClient]]
-        A callable that instantiates a NumPyClient. (default: None)
-    client : Optional[flwr.client.NumPyClient]
+    client : flwr.client.NumPyClient
         An implementation of the abstract base class `flwr.client.NumPyClient`.
     grpc_max_message_length : int (default: 536_870_912, this equals 512MB)
         The maximum length of gRPC messages that can be exchanged with the
@@ -248,42 +246,40 @@ def start_numpy_client(
     --------
     Starting a client with an insecure server connection:
 
-    >>> def client_fn(cid: str):
-    >>>     return FlowerClient()
-    >>>
     >>> start_numpy_client(
     >>>     server_address=localhost:8080,
-    >>>     client_fn=client_fn,
+    >>>     client=FlowerClient(),
     >>> )
 
     Starting an SSL-enabled gRPC client:
 
     >>> from pathlib import Path
-    >>> def client_fn(cid: str):
-    >>>     return FlowerClient()
-    >>>
     >>> start_numpy_client(
     >>>     server_address=localhost:8080,
-    >>>     client_fn=client_fn,
+    >>>     client=FlowerClient(),
     >>>     root_certificates=Path("/crts/root.pem").read_bytes(),
     >>> )
     """
-    # Start
-    _check_actionable_client(client, client_fn)
-
-    wrp_client = client.to_client() if client else None
-    wrp_clientfn = None
-    if client_fn:
+    warnings.warn(
+        "flwr.client.start_numpy_client() is deprecated and will "
+        "be removed in a future version of Flower. Instead, pass "
+        "your client to `flwr.client.start_client()` by calling "
+        "first the `.to_client()` method as shown below: \n"
+        "\tflwr.client.start_client(\n"
+        "\t\tserver_address='<IP>:<PORT>',\n"
+        "\t\tclient=FlowerClient().to_client()\n"
+        "\t)",
+        DeprecationWarning,
+        stacklevel=2,
+    )
 
-        def convert(cid: str) -> Client:
-            """Convert `NumPyClient` to `Client` upon instantiation."""
-            return client_fn(cid).to_client()
+    # Calling this function is deprecated. A warning is thrown.
+    # We first need to convert either the supplied client to `Client.`
 
-        wrp_clientfn = convert
+    wrp_client = client.to_client()
 
     start_client(
         server_address=server_address,
-        client_fn=wrp_clientfn,
         client=wrp_client,
         grpc_max_message_length=grpc_max_message_length,
         root_certificates=root_certificates,
diff --git a/src/py/flwr/client/app_test.py b/src/py/flwr/client/app_test.py
index 6d1df4697a61..7ef6410debad 100644
--- a/src/py/flwr/client/app_test.py
+++ b/src/py/flwr/client/app_test.py
@@ -17,7 +17,6 @@
 
 from typing import Dict, Tuple
 
-from flwr.client import ClientLike, to_client
 from flwr.common import (
     Config,
     EvaluateIns,
@@ -83,26 +82,18 @@ def evaluate(
 
 def test_to_client_with_client() -> None:
     """Test to_client."""
-    # Prepare
-    client_like: ClientLike = PlainClient()
-
-    # Execute
-    actual = to_client(client_like=client_like)
+    client = PlainClient().to_client()
 
     # Assert
-    assert isinstance(actual, Client)
+    assert isinstance(client, Client)
 
 
 def test_to_client_with_numpyclient() -> None:
     """Test fit_clients."""
-    # Prepare
-    client_like: ClientLike = NeedsWrappingClient()
-
-    # Execute
-    actual = to_client(client_like=client_like)
+    client = NeedsWrappingClient().to_client()
 
     # Assert
-    assert isinstance(actual, Client)
+    assert isinstance(client, Client)
 
 
 def test_start_client_transport_invalid() -> None:
diff --git a/src/py/flwr/client/message_handler/message_handler.py b/src/py/flwr/client/message_handler/message_handler.py
index b64158ea3a6c..d2eecb83d71a 100644
--- a/src/py/flwr/client/message_handler/message_handler.py
+++ b/src/py/flwr/client/message_handler/message_handler.py
@@ -28,9 +28,8 @@
     get_server_message_from_task_ins,
     wrap_client_message_in_task_res,
 )
-from flwr.client.numpy_client_wrapper import to_client
 from flwr.client.secure_aggregation import SecureAggregationHandler
-from flwr.client.typing import ClientFn, ClientLike
+from flwr.client.typing import ClientFn
 from flwr.common import serde
 from flwr.proto.task_pb2 import SecureAggregation, Task, TaskIns, TaskRes
 from flwr.proto.transport_pb2 import ClientMessage, Reason, ServerMessage
@@ -64,8 +63,7 @@ def handle(client_fn: ClientFn, task_ins: TaskIns) -> Tuple[TaskRes, int, bool]:
     server_msg = get_server_message_from_task_ins(task_ins, exclude_reconnect_ins=False)
     if server_msg is None:
         # Instantiate the client
-        client_like: ClientLike = client_fn("-1")
-        client = to_client(client_like)
+        client = client_fn("-1")
         # Secure Aggregation
         if task_ins.task.HasField("sa") and isinstance(
             client, SecureAggregationHandler
@@ -120,8 +118,7 @@ def handle_legacy_message(
         return disconnect_msg, sleep_duration, False
 
     # Instantiate the client
-    client_like: ClientLike = client_fn("-1")
-    client = to_client(client_like)
+    client = client_fn("-1")
     # Execute task
     if field == "get_properties_ins":
         return _get_properties(client, server_msg.get_properties_ins), 0, True
diff --git a/src/py/flwr/client/numpy_client_wrapper.py b/src/py/flwr/client/numpy_client_wrapper.py
deleted file mode 100644
index cfdfb6cf607c..000000000000
--- a/src/py/flwr/client/numpy_client_wrapper.py
+++ /dev/null
@@ -1,27 +0,0 @@
-# Copyright 2023 Flower Labs GmbH. All Rights Reserved.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-# ==============================================================================
-"""Wrapper for NumPyClient objects."""
-
-from flwr.client.typing import ClientLike
-
-from .client import Client
-from .numpy_client import NumPyClient, _wrap_numpy_client
-
-
-def to_client(client_like: ClientLike) -> Client:
-    """Take any Client-like object and return it as a Client."""
-    if isinstance(client_like, NumPyClient):
-        return _wrap_numpy_client(client=client_like)
-    return client_like
diff --git a/src/py/flwr/client/typing.py b/src/py/flwr/client/typing.py
index 07d1da074f1b..7ee6f069768c 100644
--- a/src/py/flwr/client/typing.py
+++ b/src/py/flwr/client/typing.py
@@ -14,10 +14,8 @@
 # ==============================================================================
 """Custom types for Flower clients."""
 
-from typing import Callable, Union
+from typing import Callable
 
 from .client import Client as Client
-from .numpy_client import NumPyClient as NumPyClient
 
-ClientLike = Union[Client, NumPyClient]
-ClientFn = Callable[[str], ClientLike]
+ClientFn = Callable[[str], Client]
diff --git a/src/py/flwr/simulation/app.py b/src/py/flwr/simulation/app.py
index 6e169493f5c8..0bb9290b6911 100644
--- a/src/py/flwr/simulation/app.py
+++ b/src/py/flwr/simulation/app.py
@@ -93,7 +93,7 @@ def start_simulation(
     client_fn : ClientFn
         A function creating client instances. The function must take a single
         `str` argument called `cid`. It should return a single client instance
-        of type ClientLike. Note that the created client instances are ephemeral
+        of type Client. Note that the created client instances are ephemeral
         and will often be destroyed after a single method invocation. Since client
         instances are not long-lived, they should not attempt to carry state over
         method invocations. Any state required by the instance (model, dataset,
diff --git a/src/py/flwr/simulation/ray_transport/ray_actor.py b/src/py/flwr/simulation/ray_transport/ray_actor.py
index 63323f51368a..e6ddfd001ff6 100644
--- a/src/py/flwr/simulation/ray_transport/ray_actor.py
+++ b/src/py/flwr/simulation/ray_transport/ray_actor.py
@@ -26,8 +26,9 @@
 from ray.util.actor_pool import ActorPool
 
 from flwr import common
-from flwr.client import Client, ClientFn, to_client
+from flwr.client import Client, ClientFn
 from flwr.common.logger import log
+from flwr.simulation.ray_transport.utils import check_clientfn_returns_client
 
 # All possible returns by a client
 ClientRes = Union[
@@ -65,9 +66,8 @@ def run(
         # return also cid which is needed to ensure results
         # from the pool are correctly assigned to each ClientProxy
         try:
-            # Instantiate client
-            client_like = client_fn(cid)
-            client = to_client(client_like=client_like)
+            # Instantiate client (check 'Client' type is returned)
+            client = check_clientfn_returns_client(client_fn(cid))
             # Run client job
             job_results = job_fn(client)
         except Exception as ex:
diff --git a/src/py/flwr/simulation/ray_transport/ray_client_proxy.py b/src/py/flwr/simulation/ray_transport/ray_client_proxy.py
index 0365cce073b4..c4fc311b48f4 100644
--- a/src/py/flwr/simulation/ray_transport/ray_client_proxy.py
+++ b/src/py/flwr/simulation/ray_transport/ray_client_proxy.py
@@ -22,7 +22,7 @@
 import ray
 
 from flwr import common
-from flwr.client import Client, ClientFn, ClientLike, to_client
+from flwr.client import Client, ClientFn
 from flwr.client.client import (
     maybe_call_evaluate,
     maybe_call_fit,
@@ -275,5 +275,5 @@ def launch_and_evaluate(
 
 def _create_client(client_fn: ClientFn, cid: str) -> Client:
     """Create a client instance."""
-    client_like: ClientLike = client_fn(cid)
-    return to_client(client_like=client_like)
+    # Materialize client
+    return client_fn(cid)
diff --git a/src/py/flwr/simulation/ray_transport/ray_client_proxy_test.py b/src/py/flwr/simulation/ray_transport/ray_client_proxy_test.py
index 8c0aaee48af0..35a082678058 100644
--- a/src/py/flwr/simulation/ray_transport/ray_client_proxy_test.py
+++ b/src/py/flwr/simulation/ray_transport/ray_client_proxy_test.py
@@ -40,9 +40,9 @@ def __init__(self, cid: str) -> None:
         self.cid = int(cid)
 
 
-def get_dummy_client(cid: str) -> DummyClient:
-    """Return a DummyClient."""
-    return DummyClient(cid)
+def get_dummy_client(cid: str) -> Client:
+    """Return a DummyClient converted to Client type."""
+    return DummyClient(cid).to_client()
 
 
 # A dummy workload
diff --git a/src/py/flwr/simulation/ray_transport/utils.py b/src/py/flwr/simulation/ray_transport/utils.py
index ec3130bb1044..01e2257b429e 100644
--- a/src/py/flwr/simulation/ray_transport/utils.py
+++ b/src/py/flwr/simulation/ray_transport/utils.py
@@ -15,8 +15,10 @@
 """Utilities for Actors in the Virtual Client Engine."""
 
 import traceback
+import warnings
 from logging import ERROR
 
+from flwr.client import Client
 from flwr.common.logger import log
 
 try:
@@ -24,6 +26,9 @@
 except ModuleNotFoundError:
     TF = None
 
+# Display Deprecation warning once
+warnings.filterwarnings("once", category=DeprecationWarning)
+
 
 def enable_tf_gpu_growth() -> None:
     """Enable GPU memory growth to prevent premature OOM."""
@@ -55,3 +60,25 @@ def enable_tf_gpu_growth() -> None:
             log(ERROR, traceback.format_exc())
             log(ERROR, ex)
             raise ex
+
+
+def check_clientfn_returns_client(client: Client) -> Client:
+    """Warn once that clients returned in `clinet_fn` should be of type Client.
+
+    This is here for backwards compatibility. If a ClientFn is provided returning
+    a different type of client (e.g. NumPyClient) we'll warn the user but convert
+    the client internally to `Client` by calling `.to_client()`.
+    """
+    if not isinstance(client, Client):
+        mssg = (
+            " Ensure your client is of type `Client`. Please convert it"
+            " using the `.to_client()` method before returning it"
+            " in the `client_fn` you pass to `start_simulation`."
+            " We have applied this conversion on your behalf."
+            " Not returning a `Client` might trigger an error in future"
+            " versions of Flower."
+        )
+
+        warnings.warn(mssg, DeprecationWarning, stacklevel=2)
+        client = client.to_client()
+    return client