Theresa May has told MPs she will seek to re-open negotiations with the EU over the Northern Ireland backstop.
The PM said she would go back to Brussels to get a “significant and legally binding change” to the controversial proposal, which aims to stop the return of border checks.
The EU has said it will not change the legal text agreed with the UK PM.
Mrs May said she knew there was a “limited appetite” in the EU, but she believed she could “secure” it.
She is expected to have phone calls with key EU leaders throughout the day ahead of a series of Commons votes over the future direction of Brexit.
Mrs May said the vote later would be a chance to “send a clear message” to EU on the backstop.
Senior Brexiteer rebels – who voted down her deal last month – have indicated they would be willing to back the rest of the UK-EU Brexit deal if she gets legal changes to the backstop.
The backstop is the insurance policy in Mrs May’s plan to prevent checks on good and people returning to the border between Northern Ireland and Ireland, which some MPs fear could leave the UK tied to the EU’s rules indefinitely.
MPs put forward a string of amendments to modify the prime minister’s Brexit plan after it was voted down by an historic margin on 15 January.
Speaker of the House, John Bercow, has named seven amendments to be debated and voted on, including one from Tory MP Sir Graham Brady calling on “alternative arrangements” to the backstop, and one from Labour MP Yvette Cooper, which could delay the exit date by up to nine months.
Opening the debate, Mrs May said: “This House has left no-one in any doubt about what it does not want. Today we need to send an emphatic message about what we do want.”