LONDON - British Prime Minister Theresa May suffered a bruising defeat in the House of Commons Wednesday night when a group of her own Conservative MPs rebelled in a crucial vote on her Brexit withdrawal bill.

She lost by just four votes, winning 305 votes against the 309 who voted against the government.

Led by former Conservative attorney general Dominic Grieve, the rebel MPs were supported by the main opposition Labour MPs to win a call for Britain's final deal on its withdrawal from the European Union (EU) to be approved by a statue passed by the Houses of Parliament.

They insisted their move was not an attempt to block Britain's withdrawal from the EU, but to stand up for the sovereignty of the British parliament.

The defeat came during the latest debate on the EU Withdrawal Bill, a key part of the government's Brexit strategy, currently going through the House of Commons.

Grieve defended his rebellion against the government, saying it was "his time to stand up and be counted".

He rejected a last-minute intervention by Government minister Dominic Rabb that had aimed to avert the rebellion, saying it had come too late.

Earlier, May insisted the parliament would have a "meaningful vote" on the terms of the final Brexit deal before Britain leaves the EU.

Immediately after the vote, a government spokesperson said in a statement that the government was disappointed that the 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," the spokesperson 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 fulfils its vital purpose."