Apparently Labour will back a referendum on the final deal...if their members vote for it. They should do, Labour members are overwhelmingly against Brexit, so if they could appeal to the Tory rebels, they could force the issue.
But that would only be a referendum on the terms of Brexit, not Brexit itself. And nothing will stop this now, we are fully committed to jumping off this cliff. Even the revelations of illegal spending by the Leave campaign (in a concerted campaign designed to avoid financial accountability) and the involvement of Russian figures in the Leave campaign, let alone the issues of implementing Brexit without fucking us all over, has not slowed down the process one bit.