diff --git a/Makefile b/Makefile
index dddaf4f287..5caa3b2ed0 100644
--- a/Makefile
+++ b/Makefile
@@ -342,6 +342,10 @@ all::
 # Define DEFAULT_HELP_FORMAT to "man", "info" or "html"
 # (defaults to "man") if you want to have a different default when
 # "git help" is called without a parameter specifying the format.
+#
+# Define TEST_GIT_INDEX_VERSION to 2, 3 or 4 to run the test suite
+# with a different indexfile format version.  If it isn't set the index
+# file format used is index-v[23].
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -2222,6 +2226,9 @@ endif
 ifdef GIT_PERF_MAKE_OPTS
 	@echo GIT_PERF_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_OPTS)))'\' >>$@
 endif
+ifdef TEST_GIT_INDEX_VERSION
+	@echo TEST_GIT_INDEX_VERSION=\''$(subst ','\'',$(subst ','\'',$(TEST_GIT_INDEX_VERSION)))'\' >>$@
+endif
 
 ### Detect Python interpreter path changes
 ifndef NO_PYTHON
diff --git a/t/t2104-update-index-skip-worktree.sh b/t/t2104-update-index-skip-worktree.sh
index 1d0879be06..29c1fb10ca 100755
--- a/t/t2104-update-index-skip-worktree.sh
+++ b/t/t2104-update-index-skip-worktree.sh
@@ -7,6 +7,8 @@ test_description='skip-worktree bit test'
 
 . ./test-lib.sh
 
+test_set_index_version 3
+
 cat >expect.full <<EOF
 H 1
 H 2
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index aeae3ca769..0bf1e63ab5 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -32,6 +32,11 @@ test_set_editor () {
 	export EDITOR
 }
 
+test_set_index_version () {
+    GIT_INDEX_VERSION="$1"
+    export GIT_INDEX_VERSION
+}
+
 test_decode_color () {
 	awk '
 		function name(n) {
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 1531c241c0..569b52dc0f 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -108,6 +108,12 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR
 
+if test -n "${TEST_GIT_INDEX_VERSION:+isset}"
+then
+	GIT_INDEX_VERSION="$TEST_GIT_INDEX_VERSION"
+	export GIT_INDEX_VERSION
+fi
+
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
 if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||