diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index 18a2993fb8..c9799a38ec 100644
--- a/modules/templates/helper.go
+++ b/modules/templates/helper.go
@@ -63,7 +63,7 @@ func NewFuncMap() template.FuncMap {
 
 		// -----------------------------------------------------------------
 		// time / number / format
-		"FileSize":      base.FileSize,
+		"FileSize":      FileSizePanic,
 		"CountFmt":      base.FormatNumberSI,
 		"TimeSince":     timeutil.TimeSince,
 		"TimeSinceUnix": timeutil.TimeSinceUnix,
@@ -249,3 +249,7 @@ func Eval(tokens ...any) (any, error) {
 	n, err := eval.Expr(tokens...)
 	return n.Value, err
 }
+
+func FileSizePanic(s int64) string {
+	panic("Usage of FileSize in templates is deprecated in Forgejo. Locale.TrSize should be used instead.")
+}