UK PM Theresa May is due at a summit in Brussels, Belgium, just hours after Conservative rebels defeated the government in a key Brexit vote.
UK MPs backed an amendment giving them a legal guarantee of a vote on the final Brexit deal struck with Brussels.
One rebel, Stephen Hammond, was sacked by the prime minister as a party vice-chairman in the aftermath of the vote.
May suffered her first defeat as prime minister by just four votes, as MPs backed an amendment to the EU Withdrawal Bill by 309 to 305.
Labour leader Jeremy Corbyn said it was a “humiliating loss of authority” for the prime minster.
The EU Withdrawal Bill is a key part of the UK government’s exit strategy. Its effects include ending the supremacy of EU law and copying existing EU law into UK law, so the same rules and regulations apply on Brexit day.
MPs have made hundreds of attempts to change the wording of the bill, but this is the first time one has succeeded.
Unless the government manages to overturn it further down the line, it means a new Act of Parliament will have to be passed before ministers can implement the withdrawal deal struck with Brussels.
May will urge European Union leaders to approve an agreement to move Brexit talks on to a second phase, describing it as a fair deal that offers a good base for a discussion of future ties.
The negotiations are expected to focus on agreeing a temporary arrangement that will begin as soon as the UK leaves the EU in March 2019.
