London - Three leading pro-EU lawmakers from Theresa May's Conservatives have resigned from the party, dealing a blow to the embattled British prime minister ahead of her trip to Brussels later Wednesday.

May's reaction

May said she was saddened by the resignations of Conservatives Heidi Allen, Anna Soubry and Sarah Wollaston but insisted that she is "doing the right thing" by continuing with her plan.

"I am saddened by this decision," May said in a statement.

"Of course, the UK's membership of the EU has been a source of disagreement both in our party and our country for a long time," she said.

"Ending that membership after four decades was never going to be easy.

"But by delivering on our manifesto commitment and implementing the decision of the British people [in the 2016 Brexit referendum], we are doing the right thing for our country," May said.

Independent Group

Allen, Soubry and Wollaston said in a joint letter to May that they felt they could no longer remain in "the party of a government whose policies and priorities are so firmly in the grip of the ERG and DUP."

The trio referred to the European Research Group of some 80 eurosceptic Conservative lawmakers and to Northern Ireland's Democratic Unionist Party, whose 10 lawmakers have propped up May's minority government since June 2017.

They plan to remain in parliament as independents and cooperate "in the centre ground of British politics" with a new Independent Group formed by eight pro-EU Labour lawmakers who resigned from their party earlier in the week.

"Brexit has redefined the Conservative Party - undoing all the efforts to modernize it," Allen, Soubry and Wollaston wrote.

"There has been a dismal failure to stand up to the hard-line ERG which operates openly as a party within a party, with its own leader, whip and policy," they said.

The trio sat with the eight ex-Labour members in parliament as May answered her weekly prime minister's questions.

The Independent Group welcomed them.

"Both our parties are broken," the group tweeted. "We are going to change politics for the better."

Brexit backstop

The parliamentary reshuffle comes amid a long-drawn-out impasse over Brexit, with London and Brussels unable as yet to agree on a politically viable deal to regulate Britain's departure from the bloc.

May told parliament that she would continue to seek changes to the "backstop" arrangement designed to guarantee an open Irish border after Brexit, in her talks with European Commission President Jean-Claude Juncker in Brussels on Wednesday.

EU officials have insisted that it will not renegotiate any part of Britain's existing withdrawal agreement, including the backstop, with just five weeks until Britain is due to leave the European Union on March 29.

The British parliament voted down that agreement in January.

"The issue that I'm taking to Brussels is the one that I've been talking to EU leaders about over the last few days, and that is the concern that was expressed in this house of ensuring that we could not find ourselves in the current backstop indefinitely," May told lawmakers.

She said a "number of ways" were under discussion to alter the backstop, ahead of an expected vote on the Brexit process next week.