diff --git a/generic/tclEncoding.c b/generic/tclEncoding.c
index 2c97901598f..9d75d1e6149 100644
--- a/generic/tclEncoding.c
+++ b/generic/tclEncoding.c
@@ -3273,12 +3273,16 @@ UtfToUtf16Proc(
 		ch = UNICODE_REPLACE_CHAR;
 	    }
 	}
-	src += len;
 	if (flags & TCL_ENCODING_LE) {
 	    if (ch <= 0xFFFF) {
 		*dst++ = (ch & 0xFF);
 		*dst++ = (ch >> 8);
 	    } else {
+		if ((dst+2) > dstEnd) {
+		    /* Surrogates need 2 more bytes! Bug [66da4d4228] */
+		    result = TCL_CONVERT_NOSPACE;
+		    break;
+		}
 		*dst++ = (((ch - 0x10000) >> 10) & 0xFF);
 		*dst++ = (((ch - 0x10000) >> 18) & 0x3) | 0xD8;
 		*dst++ = (ch & 0xFF);
@@ -3289,12 +3293,18 @@ UtfToUtf16Proc(
 		*dst++ = (ch >> 8);
 		*dst++ = (ch & 0xFF);
 	    } else {
+		if ((dst+2) > dstEnd) {
+		    /* Surrogates need 2 more bytes! Bug [66da4d4228] */
+		    result = TCL_CONVERT_NOSPACE;
+		    break;
+		}
 		*dst++ = (((ch - 0x10000) >> 18) & 0x3) | 0xD8;
 		*dst++ = (((ch - 0x10000) >> 10) & 0xFF);
 		*dst++ = ((ch >> 8) & 0x3) | 0xDC;
 		*dst++ = (ch & 0xFF);
 	    }
 	}
+	src += len;
     }
     *srcReadPtr = src - srcStart;
     *dstWrotePtr = dst - dstStart;
diff --git a/tests/utfext.test b/tests/utfext.test
index 827b7e5ecc6..1ec1cd3529c 100644
--- a/tests/utfext.test
+++ b/tests/utfext.test
@@ -224,6 +224,52 @@ namespace eval utftest {
 	}
     }
 
+    proc testspacelimit {direction enc comment hexin hexout} {
+	set id $comment-[join $hexin ""]-spacelimit
+
+        # Triple the input to avoid pathological short input case where
+        # whereby nothing is written to output. The test below
+        # requires $nchars > 0
+        set hexin $hexin$hexin$hexin
+        set hexout $hexout$hexout$hexout
+
+	set flags [list start end]
+	set constraints [list testencoding]
+
+	set maxchars [llength $hexout]
+	set in [binary decode hex $hexin]
+	set out [binary decode hex $hexout]
+	set dstlen [expr {[string length $out] - 1}]; # Smaller buffer than needed
+
+	if {$direction eq "toutf"} {
+	    set cmd Tcl_ExternalToUtf
+            set str [encoding convertfrom $enc $in]
+	} else {
+	    set cmd Tcl_UtfToExternal
+            set str [encoding convertfrom $enc $out]
+	}
+
+        # Note the tests are loose because the some encoding operations will
+        # stop even there is actually still room in the destination. For example,
+        # below only one char is written though there is room in the output.
+        # % testencoding Tcl_ExternalToUtf ascii abc {start end} {} 5 nread nwritten nchars
+        # nospace {} aÿÿÿ#
+        # % puts $nread,$nwritten,$nchars
+        # 1,1,1
+        #
+
+	test $cmd-$enc-$id-[join $flags -] "$cmd - $enc - $hexin - $flags" \
+            -constraints $constraints \
+            -body {
+            lassign [testencoding $cmd $enc $in $flags {} $dstlen nread nwritten nchars] status state buf
+            list \
+                $status \
+                [expr {$nread < [string length $in]}] \
+                [expr {$nwritten <= $dstlen}] \
+                [expr {$nchars > 0 && $nchars < [string length $str]}] \
+                [expr {[string range $out 0 $nwritten-1] eq [string range $buf 0 $nwritten-1]}]
+            } -result {nospace 1 1 1 1}
+    }
 
     #
     # Basic tests
@@ -249,9 +295,14 @@ namespace eval utftest {
 
 	    # Char limits - note no fromutf as Tcl_UtfToExternal does not support it
 	    testcharlimit toutf $enc $comment $hex $utfhex
+
+            # Space limits
+            testspacelimit toutf $enc $comment $hex $utfhex
+            testspacelimit fromutf $enc $comment $utfhex $hex
 	}
     }
 
+
     # Special cases - cesu2 high and low surrogates in separate fragments
     # This will (correctly) return "ok", not "multibyte" after first frag
     testfragment toutf cesu-8 nonbmp-split-surr \