Theresa May will urge the EU to help get her Brexit deal through the Commons by agreeing legally binding changes to the controversial backstop.
On Friday, she will say the EU's actions will "have a big impact on the outcome" when MPs vote on it next week.
But Labour's Sir Keir Starmer said it was now "clear" the PM "will not be able to deliver the changes she promised to her failed Brexit deal".
The EU says the UK must come forward with new ideas to break the deadlock.
The UK is due to leave on 29 March.