diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -248,6 +248,17 @@ def _merge(repo, mynode, orig, fcd, fco,
         return True, r
     return False, 0
 
+@internaltool('union', True,
+              _("merging %s incomplete! "
+                "(edit conflicts, then use 'hg resolve --mark')\n"))
+def _iunion(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
+    """
+    Uses the internal non-interactive simple merge algorithm for merging
+    files. It will use both left and right sides for conflict regions.
+    No markers are inserted."""
+    return _merge(repo, mynode, orig, fcd, fco, fca, toolconf,
+                  files, labels, 'union')
+
 @internaltool('merge', True,
               _("merging %s incomplete! "
                 "(edit conflicts, then use 'hg resolve --mark')\n"))
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -1202,6 +1202,11 @@ Test dynamic list of merge tools only sh
       ":tagmerge"
         Uses the internal tag merge algorithm (experimental).
   
+      ":union"
+        Uses the internal non-interactive simple merge algorithm for merging
+        files. It will use both left and right sides for conflict regions. No
+        markers are inserted.
+  
       Internal tools are always available and do not require a GUI but will by
       default not handle symlinks or binary files.
   
diff --git a/tests/test-merge-internal-tools-pattern.t b/tests/test-merge-internal-tools-pattern.t
--- a/tests/test-merge-internal-tools-pattern.t
+++ b/tests/test-merge-internal-tools-pattern.t
@@ -1,5 +1,6 @@
-Make sure that the internal merge tools (internal:fail, internal:local, and
-internal:other) are used when matched by a merge-pattern in hgrc
+Make sure that the internal merge tools (internal:fail, internal:local,
+internal:union and internal:other) are used when matched by a
+merge-pattern in hgrc
 
 Make sure HGMERGE doesn't interfere with the test:
 
@@ -110,3 +111,31 @@ Merge using default tool:
   $ hg stat
   M f
 
+Merge using internal:union tool:
+
+  $ hg update -C 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo "line 4a" >>f
+  $ hg ci -Am "Adding fourth line (commit 4)"
+  $ hg update 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ echo "line 4b" >>f
+  $ hg ci -Am "Adding fourth line v2 (commit 5)"
+  created new head
+
+  $ echo "[merge-patterns]" > .hg/hgrc
+  $ echo "* = internal:union" >> .hg/hgrc
+
+  $ hg merge 3
+  merging f
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ cat f
+  line 1
+  line 2
+  third line
+  line 4b
+  line 4a