British Prime Minister Theresa May has urged MPs to take "a second look" at her deal to leave the European Union, warning them that voting it down could open the way for the break up of the United Kingdom.

"I say to members on all sides of this house, whatever you may have previously concluded, over these next 24 hours, give this deal a second look," Mrs May told parliament.

Speaking in the House of Commons, she also said she does not believe that the 29 March Brexit date should be delayed.

Meanwhile, Labour opposition leader Jeremy Corbyn said Mrs May has "completely and utterly failed" to allay MPs' concerns about her Brexit deal.

Mr Corbyn, also speaking in the Commons, said letters between the EU and Mrs May over the backstop were "nothing more than a repetition of exactly the same position that was pulled more than one month ago".

He said: "It categorically does not give the legal assurances this House was promised and contains nothing but warm words and aspirations."

Earlier, Mrs May warned that Britain's planned exit from the European Union could be derailed by a last-ditch effort to win over Brexit-supporting politicians who have repeatedly said they will vote down her divorce deal.

The fate of the United Kingdom's exit from the EU is deeply uncertain as the House of Commons is likely to reject Mrs May's deal tomorrow.

Such a move would open up outcomes ranging from a disorderly divorce to reversing Brexit altogether.

Mrs May and EU leaders exchanged letters this morning giving assurances on her withdrawal agreement, though there was little sign of a change of heart among pro-Brexit MPs.

Speaking at a china factory in the leave-supporting city of Stoke-on-Trent, Mrs May said politicians blocking Brexit altogether was now a more likely outcome than Britain leaving without a deal.

"There are some in Westminster who would wish to delay or even stop Brexit and who will use every device available to them to do so," she said.

"While no-deal remains a serious risk, having observed the events at Westminster over the last seven days, it's now my judgment that the more likely outcome is a paralysis in parliament that risks there being no Brexit."

Yesterday, Mrs May warned politicians that failing to deliver Brexit would be "catastrophic" for democracy, and her ministers said that thwarting the outcome of the 2016 referendum could lead to rise in far-right populism.

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As part of the effort to get the deal approved by the British parliament, the EU and Mrs May set out some assurances in a choreographed exchange of letters this morning.

The EU told Mrs May that it stood by commitments to find ways to avoid triggering the controversial "Irish backstop" in their Brexit deal and that this pledge had legal weight.

In a joint reply to questions from Mrs May, European Commission President Jean-Claude Juncker and European Council President Donald Tusk said the EU stood by its commitment to try and reach a post-Brexit trade deal by the end of next year in order to avoid using the unpopular backstop.

Mrs May said the assurances might not go far enough for some MPs and the DUP has said it was insufficient.

"The letter isn't legally binding," Democratic Unionist Party deputy leader Nigel Dodds told BBC radio.

But with Mrs May's deal facing opposition from all sides in the House of Commons the letters are unlikely to change the fundamental outcome of the vote.