diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -37,6 +37,30 @@
         filename = '%s-%s' % (filename, repo.filtername)
     return filename
 
+def hasbranch(repo, branchname):
+    """check whether a branchname exists in the repo or not by reading the
+    branchmap cache"""
+
+    if not repo.cachevfs.exists(_filename(repo)):
+        # branchmap file is not present, let's go repo.branchmap() route which
+        # will create that file
+        return branchname in repo.branchmap()
+
+    # TODO: implement binary-search here for faster search
+    with repo.cachevfs(_filename(repo)) as f:
+        f = repo.cachevfs(_filename(repo))
+        lineiter = iter(f)
+        next(lineiter).rstrip('\n').split(" ", 2)
+        for l in lineiter:
+            l = l.rstrip('\n')
+            if not l:
+                continue
+            label = l.split(" ", 2)[2]
+            label = encoding.tolocal(label.strip())
+            if label == branchname:
+                return True
+    return False
+
 def read(repo):
     f = None
     try:
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1528,7 +1528,7 @@
         return scmutil.revsymbol(self, key).node()
 
     def lookupbranch(self, key):
-        if key in self.branchmap():
+        if branchmap.hasbranch(self, key):
             return key
 
         return scmutil.revsymbol(self, key).branch()
diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -11,6 +11,7 @@
 
 from .i18n import _
 from . import (
+    branchmap,
     dagop,
     destutil,
     diffutil,
@@ -499,7 +500,7 @@
         if kind == 'literal':
             # note: falls through to the revspec case if no branch with
             # this name exists and pattern kind is not specified explicitly
-            if pattern in repo.branchmap():
+            if branchmap.hasbranch(repo, pattern):
                 return subset.filter(lambda r: matcher(getbranch(r)),
                                      condrepr=('<branch %r>', b))
             if b.startswith('literal:'):