London - British Prime Minister Theresa May has seen her mandate to renegotiate the Brexit deal with Brussels weakened after she has lost another parliamentary vote.

Eurosceptics

Lawmakers in the Commons, parliament's main elected house, voted by 303 to 258 against the government's motion supporting May's plans to seek last-minute changes to Britain's EU withdrawal agreement.

A group of eurosceptics in May's Conservative party abstained, condemning her minority government to defeat.

The eurosceptics were unhappy that the motion appeared to rule out the possibility of Britain leaving the EU without a deal on March 29 if no deal has been finalized by then.

They claimed that ruling out a no-deal Brexit would weaken May's negotiating position as she attempts to win concessions on a controversial "backstop" protocol to guarantee an open Irish border.

"Malthouse Compromise"

"What we do know tonight is that there is a majority in this house for replacing the backstop with alternative arrangements," Steve Baker, deputy chair of the European Research Group of some 80 Conservative eurosceptic lawmakers, said following the vote.

Baker said achieving that majority would depend on the government accepting his group's "Malthouse Compromise," an outline proposal to extend the Brexit transition period and revise the backstop while negotiating a new free-trade agreement with the European Union.

The alternative Conservative plan has also been backed by some pro-EU lawmakers in the party and by Northern Ireland's Democratic Unionist Party (DUP), whose 10 lawmakers have propped up May's minority government since June 2017.

The parliamentary majority

Lawmakers passed an amendment last month that commits May to renegotiating the backstop. That vote followed a crushing defeat for May in a crucial vote on the Brexit deal.

She has returned to Brussels for more talks, with little sign of progress, appealing to parliament for more time to negotiate changes to the controversial backstop.

Keir Starmer, shadow Brexit secretary for the opposition Labour party, has said the defeat meant that May can "no longer claim to have a 'substantial and sustainable majority' in parliament."

Starmer was referring to Brussels' insistence that May must show a stable parliamentary majority supporting her before it will consider any concessions. "Parliament must decide what happens next," Starmer said.

Six weeks before Brexit

But Liz Truss, a junior minister in May's government, said the "fundamentals haven't changed" following the vote.

"Still a majority for PM's deal with changes to the backstop," Truss tweeted.

Brexit Secretary Stephen Barclay said in the debate before the vote that it was still the government's priority to "address the indefinite nature of the backstop" and to secure a deal, just six weeks before Britain is due to withdraw from the European Union on March 29.

Barclay suggested the government remained unwilling to postpone that exit date by extending the two-year Brexit negotiating period, but he warned of the economic risks if the country crashes out without a deal.

Starmer accused May earlier of "pretending to make progress while running down the clock."

He presented a Labour amendment to put a "hard stop" to the Brexit process and allow parliament to take control if she fails to present a deal for a vote by February 27.

The amendment was defeated by 322 to 306 votes.