diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -278,6 +278,11 @@ def checknewlabel(repo, lbl, kind):
             )
     try:
         int(lbl)
+        if b'_' in lbl:
+            # If label contains underscores, Python might consider it an
+            # integer (with "_" as visual separators), but we do not.
+            # See PEP 515 - Underscores in Numeric Literals.
+            raise ValueError
         raise error.InputError(_(b"cannot use an integer as a name"))
     except ValueError:
         pass
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -82,6 +82,11 @@ trailing or leading spaces should be str
   (use 'hg update' to switch to it)
   [10]
 
+underscores in numeric branch names (issue6737)
+
+  $ hg branch 2700_210
+  marked working directory as branch 2700_210
+
 verify update will accept invalid legacy branch names
 
   $ hg init test-invalid-branch-name