Ex-foreign secretary Boris Johonson says winning a "freedom clause" would be "unadulterated good Brexit news".
MPs on Tuesday will vote on a series of amendments of MP's plans that could shape the fiture direction of Brexit.
But the Irish deputy PM says changes to the backstop-aimed at preventing a hard border-would not be acceptable.
The backstop is the "insurance policy" in the withdrawal deal, intended to ensure that whatever else happens, there wil be no return to a visible border between Northern Ireland and the Irish Republic after the UK leaves the EU.