diff --git a/mercurial/pathutil.py b/mercurial/pathutil.py
--- a/mercurial/pathutil.py
+++ b/mercurial/pathutil.py
@@ -142,3 +142,25 @@ def canonpath(root, cwd, myname, auditor
             name = dirname
 
         raise util.Abort(_("%s not under root '%s'") % (myname, root))
+
+def normasprefix(path):
+    '''normalize the specified path as path prefix
+
+    Returned vaule can be used safely for "p.startswith(prefix)",
+    "p[len(prefix):]", and so on.
+
+    For efficiency, this expects "path" argument to be already
+    normalized by "os.path.normpath", "os.path.realpath", and so on.
+
+    See also issue3033 for detail about need of this function.
+
+    >>> normasprefix('/foo/bar').replace(os.sep, '/')
+    '/foo/bar/'
+    >>> normasprefix('/').replace(os.sep, '/')
+    '/'
+    '''
+    d, p = os.path.splitdrive(path)
+    if len(p) != len(os.sep):
+        return path + os.sep
+    else:
+        return path
diff --git a/mercurial/subrepo.py b/mercurial/subrepo.py
--- a/mercurial/subrepo.py
+++ b/mercurial/subrepo.py
@@ -276,8 +276,7 @@ def reporelpath(repo):
     parent = repo
     while util.safehasattr(parent, '_subparent'):
         parent = parent._subparent
-    p = parent.root.rstrip(os.sep)
-    return repo.root[len(p) + 1:]
+    return repo.root[len(pathutil.normasprefix(parent.root)):]
 
 def subrelpath(sub):
     """return path to this subrepo as seen from outermost repo"""
diff --git a/tests/test-doctest.py b/tests/test-doctest.py
--- a/tests/test-doctest.py
+++ b/tests/test-doctest.py
@@ -19,6 +19,7 @@ testmod('mercurial.hg')
 testmod('mercurial.hgweb.hgwebdir_mod')
 testmod('mercurial.match')
 testmod('mercurial.minirst')
+testmod('mercurial.pathutil')
 testmod('mercurial.revset')
 testmod('mercurial.store')
 testmod('mercurial.subrepo')