diff --git a/src/errors.h b/src/errors.h
index 990fb86..e6d7ab2 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -351,3 +351,5 @@
 	INIT(= N_("E1156: Cannot change the argument list recursively"));
 EXTERN char e_missing_return_type[]
 	INIT(= N_("E1157: Missing return type"));
+EXTERN char e_cannot_use_flatten_in_vim9_script[]
+	INIT(= N_("E1158: Cannot use flatten() in Vim9 script"));
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f0a15f9..0b4b502 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -954,6 +954,8 @@
 			ret_string,	    f_findfile},
     {"flatten",		1, 2, FEARG_1,	    NULL,
 			ret_list_any,	    f_flatten},
+    {"flattennew",	1, 2, FEARG_1,	    NULL,
+			ret_list_any,	    f_flattennew},
     {"float2nr",	1, 1, FEARG_1,	    NULL,
 			ret_number,	    FLOAT_FUNC(f_float2nr)},
     {"floor",		1, 1, FEARG_1,	    NULL,
diff --git a/src/list.c b/src/list.c
index ca6e469..c51c99b 100644
--- a/src/list.c
+++ b/src/list.c
@@ -740,7 +740,7 @@
  * It does nothing if "maxdepth" is 0.
  * Returns FAIL when out of memory.
  */
-    static int
+    static void
 list_flatten(list_T *list, long maxdepth)
 {
     listitem_T	*item;
@@ -748,7 +748,7 @@
     int		n;
 
     if (maxdepth == 0)
-	return OK;
+	return;
     CHECK_LIST_MATERIALIZE(list);
 
     n = 0;
@@ -757,7 +757,7 @@
     {
 	fast_breakcheck();
 	if (got_int)
-	    return FAIL;
+	    return;
 
 	if (item->li_tv.v_type == VAR_LIST)
 	{
@@ -765,7 +765,7 @@
 
 	    vimlist_remove(list, item, item);
 	    if (list_extend(list, item->li_tv.vval.v_list, next) == FAIL)
-		return FAIL;
+		return;
 	    clear_tv(&item->li_tv);
 	    tofree = item;
 
@@ -787,15 +787,13 @@
 	    item = item->li_next;
 	}
     }
-
-    return OK;
 }
 
 /*
- * "flatten(list[, {maxdepth}])" function
+ * "flatten()" and "flattennew()" functions
  */
-    void
-f_flatten(typval_T *argvars, typval_T *rettv)
+    static void
+flatten_common(typval_T *argvars, typval_T *rettv, int make_copy)
 {
     list_T  *l;
     long    maxdepth;
@@ -822,10 +820,48 @@
     }
 
     l = argvars[0].vval.v_list;
-    if (l != NULL && !value_check_lock(l->lv_lock,
-				      (char_u *)N_("flatten() argument"), TRUE)
-		 && list_flatten(l, maxdepth) == OK)
-	copy_tv(&argvars[0], rettv);
+    rettv->v_type = VAR_LIST;
+    rettv->vval.v_list = l;
+    if (l == NULL)
+	return;
+
+    if (make_copy)
+    {
+	l = list_copy(l, TRUE, get_copyID());
+	rettv->vval.v_list = l;
+	if (l == NULL)
+	    return;
+    }
+    else
+    {
+	if (value_check_lock(l->lv_lock,
+				     (char_u *)N_("flatten() argument"), TRUE))
+	    return;
+	++l->lv_refcount;
+    }
+
+    list_flatten(l, maxdepth);
+}
+
+/*
+ * "flatten(list[, {maxdepth}])" function
+ */
+    void
+f_flatten(typval_T *argvars, typval_T *rettv)
+{
+    if (in_vim9script())
+	emsg(_(e_cannot_use_flatten_in_vim9_script));
+    else
+	flatten_common(argvars, rettv, FALSE);
+}
+
+/*
+ * "flattennew(list[, {maxdepth}])" function
+ */
+    void
+f_flattennew(typval_T *argvars, typval_T *rettv)
+{
+    flatten_common(argvars, rettv, TRUE);
 }
 
 /*
diff --git a/src/proto/list.pro b/src/proto/list.pro
index 7c9ddae..4ccc897 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -31,6 +31,7 @@
 int list_insert_tv(list_T *l, typval_T *tv, listitem_T *item);
 void list_insert(list_T *l, listitem_T *ni, listitem_T *item);
 void f_flatten(typval_T *argvars, typval_T *rettv);
+void f_flattennew(typval_T *argvars, typval_T *rettv);
 int list_extend(list_T *l1, list_T *l2, listitem_T *bef);
 int list_concat(list_T *l1, list_T *l2, typval_T *tv);
 list_T *list_slice(list_T *ol, long n1, long n2);
diff --git a/src/testdir/test_flatten.vim b/src/testdir/test_flatten.vim
index 34431f8..7d321f7 100644
--- a/src/testdir/test_flatten.vim
+++ b/src/testdir/test_flatten.vim
@@ -81,4 +81,13 @@
   call assert_equal([2, l:x], l:y)
 endfunc
 
+func Test_flattennew()
+  let l = [1, [2, [3, 4]], 5]
+  call assert_equal([1, 2, 3, 4, 5], flattennew(l))
+  call assert_equal([1, [2, [3, 4]], 5], l)
+
+  call assert_equal([1, 2, [3, 4], 5], flattennew(l, 1))
+  call assert_equal([1, [2, [3, 4]], 5], l)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 5fd510f..5e678fa 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -382,6 +382,23 @@
   CheckDefExecFailure(['echo findfile("")'], 'E1142:')
 enddef
 
+def Test_flattennew()
+  var lines =<< trim END
+      var l = [1, [2, [3, 4]], 5]
+      call assert_equal([1, 2, 3, 4, 5], flattennew(l))
+      call assert_equal([1, [2, [3, 4]], 5], l)
+
+      call assert_equal([1, 2, [3, 4], 5], flattennew(l, 1))
+      call assert_equal([1, [2, [3, 4]], 5], l)
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      echo flatten([1, 2, 3])
+  END
+  CheckDefAndScriptFailure(lines, 'E1158:')
+enddef
+
 def Test_fnamemodify()
   CheckDefSuccess(['echo fnamemodify(test_null_string(), ":p")'])
   CheckDefSuccess(['echo fnamemodify("", ":p")'])
diff --git a/src/version.c b/src/version.c
index 5a89933..d34839c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2449,
+/**/
     2448,
 /**/
     2447,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index a9edc80..5718efd 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2900,6 +2900,12 @@
 	idx = find_internal_func(name);
 	if (idx >= 0)
 	{
+	    if (STRCMP(name, "flatten") == 0)
+	    {
+		emsg(_(e_cannot_use_flatten_in_vim9_script));
+		goto theend;
+	    }
+
 	    if (STRCMP(name, "add") == 0 && argcount == 2)
 	    {
 		garray_T    *stack = &cctx->ctx_type_stack;