The prime minister is due in Brussels on Thursday, 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 Theresa May as Conservative 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 kick in 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 One'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."
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.
What does it mean?
BBC political editor Laura Kuenssberg
It's the first time that Theresa May has been defeated on her own business in the Commons. She has to front-up in Brussels tomorrow with other EU leaders only hours after an embarrassing loss in Parliament.
Beyond the red faces in government tonight, does it really matter? Ministers tonight are divided on that. Two cabinet ministers have told me while it's disappointing, it doesn't really matter in the big picture.
It's certainly true that the Tory party is so divided over how we leave the EU that the Parliamentary process was always going to be very, very choppy.
But another minister told me the defeat is "bad for Brexit" and was openly frustrated and worried about their colleagues' behaviour.
The EU Withdrawal Bill is a key part of the 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 been making hundreds of attempts to change its wording – 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.
Ministers had made several efforts to placate the Conservative rebels, and argued that Mr Grieve's amendment would put unnecessary time pressure on the government if talks with the EU continued until the last minute.
And minutes before the vote, they offered a last-minute promise of action at a later stage of the bill's journey through Parliament.
Some Conservatives said this had changed their minds. But Mr Grieve said it was "too late".
There was an often heated debate in the Chamber before the crunch vote on the amendment.
Critics said the rebels were trying to "frustrate" Brexit and tie the government's hands.
After the result was announced, one of the rebels, former cabinet minister Nicky Morgan, tweeted: "Tonight Parliament took control of the EU Withdrawal process."
But other Conservative MPs reacted angrily, with one, Nadine Dorries, saying the rebels should be deselected.
The Tory rebels were Mr Grieve, Heidi Allen, Ken Clarke, Jonathan Djanogly, Stephen Hammond, Sir Oliver Heald, Nicky Morgan, Bob Neill, Antoinette Sandbach, Anna Soubry and Sarah Wollaston.
Another Conservative MP, John Stevenson, abstained by voting in both lobbies.
Two Labour MPs, Frank Field and Kate Hoey, voted with the government.