diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -287,6 +287,9 @@ def _formatconflictmarker(repo, ctx, tem
     label = ('%s:' % label).ljust(pad + 1)
     mark = '%s %s' % (label, templater.stringify(templateresult))
 
+    if mark:
+        mark = mark.splitlines()[0] # split for safety
+
     # The <<< marks add 8 to the length, and '...' adds three, so max
     # length of the actual marker is 69.
     maxlength = 80 - 8 - 3
diff --git a/tests/test-conflict.t b/tests/test-conflict.t
--- a/tests/test-conflict.t
+++ b/tests/test-conflict.t
@@ -52,6 +52,26 @@ Verify custom conflict markers
   something
   >>>>>>> other: test 1
 
+Verify line splitting of custom conflict marker which causes multiple lines
+
+  $ hg up -q --clean .
+  $ cat >> .hg/hgrc <<EOF
+  > [ui]
+  > mergemarkertemplate={author} {rev}\nfoo\nbar\nbaz
+  > EOF
+
+  $ hg -q merge 1
+  warning: conflicts during merge.
+  merging a incomplete! (edit conflicts, then use 'hg resolve --mark')
+  [1]
+
+  $ cat a
+  <<<<<<< local: test 2
+  something else
+  =======
+  something
+  >>>>>>> other: test 1
+
 Verify basic conflict markers
 
   $ hg up -q --clean .