Theresa May is due at a summit in Brussels, hours after Conservative rebels in the Commons defeated the government in a key Brexit vote.
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.
Other EU member states could decide to move forward to trade talks with the UK at their two-day summit.
The negotiations are first expected to focus on agreeing a temporary arrangement that will begin as soon as the UK leaves the EU in March 2019.
On the eve of the summit, Mrs May suffered her first Commons 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 minister.
Unless it is overturned by the government at a later stage, it means MPs will get to vote on the final deal reached with Brussels before it is ratified.
The government had previously offered a vote. But critics wanted a guarantee that this would be “meaningful”, claiming the bill gave ministers the power to bypass Parliament in implementing the withdrawal agreement.
Dominic Grieve MP, who tabled the amendment, said the bill “couldn’t be allowed to stay in the condition it was in”.
The former attorney general, told BBC Two’s Newsnight: “The right thing is carrying out Brexit in an orderly, sensible way, which has a proper process to it.”
He said Parliament’s ability to interfere with Brexit negotiations was “limited”, adding: “I’ve been studious in not trying to interfere with the government’s negotiating strategy, I’ve hardly asked a question.”
former transport minister Mr Hammond denied the prime minister had been undermined by the vote.
“I think, frankly, last night was avoidable and there is dismay on all sides that it got to where it did,” he added.
Asked about calls from fellow Tory MPs to deselect rebels, he said: “I make it a cardinal rule not to be rude about my colleagues in public and I’m not going to start now.”
The government said in a statement: “We are disappointed that Parliament has voted for this amendment despite the strong assurances that we have set out.
“We are as clear as ever that this Bill, and the powers within it, are essential.
“This amendment does not prevent us from preparing our statute book for exit day. We will now determine whether further changes are needed to the Bill to ensure it fulfils its vital purpose.”
Speaking after the vote, ministers said the “minor setback” would not prevent the UK leaving the EU in 2019.