British Prime Minister Theresa May has admitted she may not garner enough support to get her twice-defeated Brexit deal through the Commons next week, amid mounting speculation about the future of her leadership.

Mrs May wrote to MPs warning that if there is insufficient support for her Withdrawal Agreement in the coming days, that she could seek an extension to Britain's EU membership beyond the European Parliament elections.

It came after the DUP - whose support will be crucial if her government is to win - indicated they would not back her deal, with the party's deputy leader Nigel Dodds lamenting her failure to secure any changes to the backstop while in Brussels.

"Nothing has changed as far as the Withdrawal Agreement is concerned. We will not accept any deal which poses a long-term risk to the constitutional and economic integrity of the United Kingdom," he said.

Mrs May sought to soften her approach in her letter to parliamentarians, offering to hold talks with MPs and thanking those who have backed her plan previously.

And she laid out the choices available to the Commons after she agreed a delay to Brexit with EU leaders in Brussels on Thursday night.

She said the UK could revoke Article 50; leave without a deal; ask for an extension beyond 12 April if her deal is rejected or not voted on; or leave on 22 May if it is finally approved.

Mrs May wrote: "If it appears that there is not sufficient support to bring the deal back next week, or the House rejects it again, we can ask for another extension before 12 April - but that will involve holding European Parliament elections.

"If it appears that there is sufficient support and the Speaker permits it, we can bring the deal back next week and if it is approved we can leave on 22 May."

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It came as ministers moved to try to head off an attempt by MPs to seize control of Commons business in a bid to secure a "softer" Brexit.

A cross-party group of pro-EU MPs claimed they had the numbers to force a series of "indicative votes" on alternatives to Mrs May's Brexit deal.

But Business Secretary Greg Clark said the government was prepared to enable Parliament to express a view on the various options if Mrs May's deal is rejected by the Commons for a third time next week.

Defeat for the Government on Monday on the amendment - tabled by former ministers Sir Oliver Letwin and Dominic Grieve and Labour MP Hilary Benn - would be a further humiliation for Mrs May.

If the amendment is passed, it would pave the way for a series of "indicative votes" in the House on Wednesday, effectively taking control of the Brexit process out of the hands of the Government.

It comes as hundreds of thousands of supporters of a second referendum will march through central London today, where Labour's deputy leader Tom Watson is expected to say he will vote for the PM's deal if it is put to the people.