In this grab taken from video, Britain's Prime Minister Theresa May speaks during Prime Minister's Questions in the House of Commons, London. Picture: House of Commons/PA via AP

London - Prime Minister Theresa May has told lawmakers from her Conservative party that she will step down once a deal for Britain to leave the European Union is finalised.

"I know there is a desire for a new approach - and new leadership - in the second phase of the Brexit negotiations - and I won't stand in the way of that," May told the party's 1922 Committee of lawmakers, according to excerpts of her speech released by Downing Street.

"I know some people are worried that if you vote for the withdrawal agreement, I will take that as a mandate to rush on into phase two without the debate we need to have. I won't - I hear what you are saying," she said.

"But we need to get the deal through and deliver Brexit."

Lawmakers in the Commons, parliament's elected main house, opened a debate on possible alternatives to May's deal earlier Wednesday, after she failed to halt the voting process.

Many eurosceptic Conservatives had urged May to resign this week or set a date to step down in exchange for them backing her unpopular deal, which parliament has rejected twice.

"So hard Brexiteers will vote for the PM's 'deal' not because it's good for our country and the right thing to do - not even because it delivers Brexit, but because it gets rid of the PM #Shameful," tweeted pro-EU lawmaker Anna Soubry, who resigned from the Conservatives over May's Brexit policy.

Scottish National Party leader Nicola Sturgeon, who advocates remaining in the EU, suggested that May's agreement to step down earlier than expected might make it harder for those fighting to soften or stop Brexit.

"If Brexit ends up being forced through on the basis of a deal no one supports - indeed a deal so bad that the PM has to promise to resign to get it through - it will make an already bad project even worse," Sturgeon tweeted.

The lawmakers voted by 331 to 287 in favour of a motion to allow the Commons to hold non-binding "indicative" votes later Wednesday. May's government opposed the motion, saying the indicative votes would set a "worrying precedent."

In more bad news for May, Commons speaker John Bercow repeated his warning last week that he might not put any new motion on May's Brexit deal to a vote unless it differs significantly from the one that lawmakers overwhelmingly rejected twice.

Bercow selected eight of 16 motions on Brexit alternatives for voting on Wednesday.

The options include a second referendum, leaving the EU without a deal and keeping Britain in a much closer relationship with the EU than the one negotiated by May.

A second round of voting is expected on Monday, but May has refused to commit her government to acting on the results.

Many eurosceptics were angered by May's decision to request a short delay to Brexit last week.

Britain must decide by April 12 if it is to request a longer-term Brexit extension. But if it does so, it would have to participate in the EU elections.