London (CNN)UK Prime Minister Theresa May headed to Brussels on Thursday for a major summit of EU leaders, just hours after her government suffered an embarrassing defeat in Parliament in a vote on key Brexit legislation.
Lawmakers backed an amendment to the Brexit bill that means the UK Parliament must be given a vote on the final deal with the European Union before withdrawal begins.
The narrow defeat -- by 309 to 305 votes -- was May's first as Prime Minister and leaves her in a weakened position at home as she travels to an EU Council meeting in the Belgian capital, during which the bloc's other 27 leaders are set to confirm that Brexit negotiations can move on to to the next stage.
May's setback came less than a week after the UK and EU secured a significant breakthrough in Brexit negotiations.
That breakthrough, in which the two sides agreed on the terms for Britain's withdrawal, opened the door for the two parties to progress onto a second phase that will include discussions over a transition period and the nature of a future trading relationship -- both crucial to Britain's economy.
May, who leads a minority Conservative government since a snap election called last summer, has had to battle bitter divisions within her own party over Brexit -- as well as pressure from opposition parties and business leaders -- as she seeks to negotiate Britain's departure from the EU.
Arriving in Brussels for the two-day summit, European Council President Donald Tusk said: "Tomorrow we will end formally our first phase of the Brexit negotiations.
"Both these achievements demanded courage, realism, but above all our unity. By the way, I have no doubts, the real test of our unity will be the second phase of the Brexit talks."
May and European Commission President Jean-Claude Juncker reached a milestone in Brexit talks last week.
'Meaningful vote'
Amendment seven to clause nine of the EU withdrawal bill was put forward by Conservative MP Dominic Grieve, who led a faction of rebels within May's party who demanded a "meaningful vote" on the deal reached with Brussels before it is ratified.
UK, EU reach breakthrough Brexit agreement 01:53
The bill is a key piece of legislation that aims to transfer EU law into UK law upon the country's planned departure from the bloc on March 29, 2019.
On Wednesday morning, as the government sought to head off the rebellion, Brexit Secretary David Davis submitted a written statement to the House of Commons in which he promised lawmakers a vote on the deal. The government then offered another concession later in the Commons debate.
But the small group of Conservative rebels stuck to their guns to vote alongside opposition Labour and Liberal Democrat lawmakers in favor of the amendment, which gives a legal guarantee that Parliament will be able to vote on the deal before it comes into effect.
Grieve had argued that without the amendment, the bill would give ministers powers to implement the deal without proper scrutiny from lawmakers.
A spokesman for the government said it was disappointed that Parliament had voted for the bill despite the "strong assurances" it had offered.
"We are as clear as ever that this bill, and the powers within it, are essential," he said. "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 fulfills its vital purpose."
Corbyn: 'Humiliating loss'
Labour leader Jeremy Corbyn described the defeat as "a humiliating loss of authority for the Government on the eve of the European Council meeting" in a statement on his Facebook page.
"Labour has made the case since the referendum for a meaningful vote in Parliament on the terms of Britain's withdrawal from the European Union," he wrote. "Theresa May has resisted democratic accountability. Her refusal to listen means she will now have to accept Parliament taking back control."
The government faces another potential showdown next week over an amendment of its own to the withdrawal bill that seeks to enshrine in law that the United Kingdom will leave the European Union at 11 p.m. GMT on March 29, 2019.
Critics argue that this would leave no flexibility at the end of the negotiating process.
