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.

A series of missteps

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