London(CNN) British Prime Minister Theresa May has lost a key vote on the EU Withdrawal bill, her first serious parliamentary Brexit setback.
Members of Parliament voted Wednesday for an amendment to the bill meaning lawmakers must approve the final deal with the European Union before withdrawal begins.
May lost by a close margin of 309-305 votes. Amendment 7 to Clause 9 was tabled by May's own Conservative MP Dominic Grieve, who leads a faction of "rebels" within the party.
The bill is a key piece of legislation which aims to transfer EU law into UK law upon the country's planned departure from the bloc on March 29, 2019.
The result was yet another disappointment for May, who has been struggling ever since her decision to hold a snap election in June, which resulted in the loss of her governing majority and the need to negotiate a deal with the Democratic Unionist Party of Northern Ireland -- which has 10 seats in Parliament -- to remain in a position to pass laws.
On Wednesday, her government's instability was exposed by a group of lawmakers which has constantly argued that Parliament should have the final say over the terms of the draft agreement before the UK leaves the EU.
This victory represents the first major setback for the government during the period where the bill has been making its way through Parliament with lawmakers attempting to add amendments.
Should Parliament vote against the government on any final deal, the Prime Minister could be forced to renegotiate with Brussels -- raising the prospect of the UK crashing out of the bloc without a trade deal and instead being governed by the rules of the World Trade Organization, a course of action sometimes referred to as a "Hard Brexit."
The setback comes less than a week after the UK and EU secured a significant breakthrough in Brexit negotiations, allowing the two parties to progress onto a crucial second phase that will include discussions over the nature of a future trading relationship.