Theresa May's suggested giving British MPs more of a say over the backstop to try and gain support for her Brexit deal.
The UK government has raised the possibility of Parliament choosing when and if they want to go into the measure aimed at avoiding a hard border.
But Tory backbenchers have criticised the move while DUP leader Arlene Foster said it was "legislative tinkering".
The crucial vote on whether MPs will back the deal takes place on Tuesday.
Theresa May could try to delay the vote - but constitutional expert Andrew Kennon thinks it's too late for that.