diff --git a/ChangeLog b/ChangeLog
index 0a76ccba5..bdc6c5e93 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -19,6 +19,9 @@ library installation directories.
 pattern is called recursively, which could prevent valid matches requiring
 backtracking after the recursive call.
 
+3. (#1040) Fixed the start_match field passed to interpreter callouts after \K
+resets the start of a match, including restoration on backtracking.
+
 
 Version 10.49 28-September-2026
 -------------------------------
diff --git a/src/pcre2_match.c b/src/pcre2_match.c
index e4f8f551c..f0a3735d3 100644
--- a/src/pcre2_match.c
+++ b/src/pcre2_match.c
@@ -307,15 +307,15 @@ do_callout(heapframe *F, match_block *mb, PCRE2_SIZE *lengthptr)
 
   callout_ovector = (PCRE2_SIZE *)(Fovector)-2;
 
-  /* The cb->version, cb->subject, cb->subject_length, and cb->start_match fields
-  are set externally. The first 3 never change; the last is updated for each
-  bumpalong. */
+  /* The cb->version, cb->subject, and cb->subject_length fields are set
+  externally and never change. */
 
   cb = mb->cb;
   cb->capture_top = (uint32_t)Foffset_top / 2 + 1;
   cb->capture_last = Fcapture_last;
   cb->offset_vector = callout_ovector;
   cb->mark = mb->nomatch_mark;
+  cb->start_match = (PCRE2_SIZE)(Fstart_match - mb->start_subject);
   cb->current_position = (PCRE2_SIZE)(Feptr - mb->start_subject);
   cb->pattern_position = GET(Fecode, 1);
   cb->next_item_length = GET(Fecode, 1 + LINK_SIZE);
@@ -8719,7 +8719,6 @@ pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2
     /* OK, we can now run the match. If "hitend" is set afterwards, remember the
     first starting point for which a partial match was found. */
 
-    cb.start_match = (PCRE2_SIZE)(start_match - subject);
     cb.callout_flags |= PCRE2_CALLOUT_STARTMATCH;
 
     mb->start_used_ptr = start_match;
diff --git a/testdata/testinput2 b/testdata/testinput2
index ab50b797e..72f00b7b4 100644
--- a/testdata/testinput2
+++ b/testdata/testinput2
@@ -8453,4 +8453,37 @@ a)"xI
     abc\=ps,substitute_replacement_only,replace=>$_<
     abc\=ps,substitute_replacement_only,replace=>$'<
 
+# Callout start offsets after resetting the start of a match.
+
+/(?C1)A\K(?C2)B(?C3)C/
+    _ABC_
+    _ABC_ABC_\=offset=4
+
+/A\K(?C"after")BC/
+    _ABC_
+
+/A\K(?C1)B\K(?C2)C/
+    _ABC_
+
+/A(?:B\K(?C1)D|(?C2)B)(?C3)C/
+    _ABC_
+
+/A\K(?(?C1)(?=B)B|D)C/
+    _ABC_\=no_jit
+
+/(?(DEFINE)(?<x>A\K(?C1)B))(?&x)(?C2)C/
+    _ABC_
+
+/A\K(?C1)B/
+    _AC_AB_
+
+/(?C1)A\K(?C2)B/no_start_optimize
+    _AC_AB_
+
+/(?C1)A(?C2)B(?C3)C/
+    _ABC_
+
+/A\KBC/auto_callout
+    _ABC_
+
 # End of testinput2
diff --git a/testdata/testoutput2 b/testdata/testoutput2
index 9fb4658c0..8feccd978 100644
--- a/testdata/testoutput2
+++ b/testdata/testoutput2
@@ -23955,6 +23955,94 @@ Failed: error -76 at offset 3 in replacement: replacement $' or $_ not supported
 Failed: error -76 at offset 3 in replacement: replacement $' or $_ not supported with partial match
         here: >$' |<--| <
 
+# Callout start offsets after resetting the start of a match.
+
+/(?C1)A\K(?C2)B(?C3)C/
+    _ABC_
+--->_ABC_
+  1  ^        A
+  2   ^       B
+  3   ^^      C
+ 0: BC
+    _ABC_ABC_\=offset=4
+--->_ABC_ABC_
+  1      ^        A
+  2       ^       B
+  3       ^^      C
+ 0: BC
+
+/A\K(?C"after")BC/
+    _ABC_
+Callout (7): "after"
+--->_ABC_
+      ^       B
+ 0: BC
+
+/A\K(?C1)B\K(?C2)C/
+    _ABC_
+--->_ABC_
+  1   ^       B
+  2    ^      C
+ 0: C
+
+/A(?:B\K(?C1)D|(?C2)B)(?C3)C/
+    _ABC_
+--->_ABC_
+  1    ^      D
+  2  ^^       B
+  3  ^ ^      C
+ 0: ABC
+
+/A\K(?(?C1)(?=B)B|D)C/
+    _ABC_\=no_jit
+--->_ABC_
+  1   ^       (?=
+ 0: BC
+
+/(?(DEFINE)(?<x>A\K(?C1)B))(?&x)(?C2)C/
+    _ABC_
+--->_ABC_
+  1   ^       B
+  2   ^^      C
+ 0: BC
+
+/A\K(?C1)B/
+    _AC_AB_
+--->_AC_AB_
+  1   ^         B
+  1      ^      B
+ 0: B
+
+/(?C1)A\K(?C2)B/no_start_optimize
+    _AC_AB_
+--->_AC_AB_
+  1 ^           A
+  1  ^          A
+  2   ^         B
+  1   ^         A
+  1    ^        A
+  1     ^       A
+  2      ^      B
+ 0: B
+
+/(?C1)A(?C2)B(?C3)C/
+    _ABC_
+--->_ABC_
+  1  ^        A
+  2  ^^       B
+  3  ^ ^      C
+ 0: ABC
+
+/A\KBC/auto_callout
+    _ABC_
+--->_ABC_
+ +0  ^        A
+ +1  ^^       \K
+ +3   ^       B
+ +4   ^^      C
+ +5   ^ ^     End of pattern
+ 0: BC
+
 # End of testinput2
 Error -80: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data