Speaking in a factory in Leave-voting Stoke-on-Trent, the Prime Minister said the letter from European Council president Donald Tusk and European Commission president Jean-Claude Juncker made clear that the backstop was "not a threat or a trap".

But her hopes that the letter would win over enough MPs to rescue her Withdrawal Agreement looked set to be dashed, as the Democratic Unionist Party - which props up her minority administration - dismissed it as "meaningless".

"Rather than reassure us, the Tusk and Juncker letter bolsters our concerns," said DUP deputy leader Nigel Dodds, who called on the Prime Minister to demand changes to the Agreement itself.

Theresa May meeting EU Commission president Jean-Claude Juncker at Downing Street

Brexit: Theresa May accused of 'hypocrisy' over devolution votes

The Prime Minister warned that MPs would be behaving with the "height of recklessness" if they rejected her Withdrawal Agreement in Tuesday's historic vote, when no alternative deal was on offer which was negotiable and respected the 2016 referendum result.

Mrs May said that the presidents' letter provided "valuable new clarifications and assurances" to address the concerns of MPs who fear the backstop, which is designed to prevent a hard border in Ireland, could become a permanent arrangement which the UK could leave only with approval from the EU.

She said the letter delivered a commitment from the EU to begin work on a new post-Brexit relationship as soon as the Withdrawal Agreement is ratified, and an explicit commitment that the new relationship does not have to "replicate" the backstop arrangement, under which the UK would remain in a customs union with the EU.

It also holds out the prospect of a fast-track process to bring a new free trade agreement into force, even if some of the 27 remaining members delay ratification, and allow the UK to offer Northern Ireland a "Stormont lock" on new EU laws being added to the backstop, in an attempt to win back the DUP’s support.

"The letters published today have legal force and must be used to interpret the meaning of the Withdrawal Agreement, including in any future arbitration," said the Prime Minister

"They make absolutely clear the backstop is not a threat or a trap.

"I fully understand that the new legal and political assurances which are contained in the letters from Donald Tusk and Jean-Claude Juncker do not go as far as some MPs would like.

"But I am convinced that MPs now have the clearest assurances that this is the best deal possible and that it is worthy of their support."

In their letter, Mr Tusk and Mr Juncker stressed they were "not in a position" to rewrite or amend the Withdrawal Agreement secured by Mrs May last year.

But they assured the Prime Minister that the EU "does not wish to see the backstop enter into force", as it would represent a "sub-optimal trading arrangement for both sides".

The EU wants to ensure it would "only be in place for as long as strictly necessary".

The EU presidents stated: "Were the backstop to enter into force in whole or in part, it is intended to apply only temporarily, unless and until it is superseded by a subsequent agreement."

With expectations high at Westminster that the Prime Minister is heading for a crushing defeat in Tuesday's crucial vote, Mrs May issued a plea to MPs concerned about the danger of a no-deal Brexit to back her.

"You can take no-deal off the table by voting for that deal. And if no-deal is as bad as you believe it is, it would be the height of recklessness to do anything else."

But she said that recent events meant a no-deal Brexit was less likely than "a paralysis in Parliament that risks there being no Brexit".

Warning that failure to deliver Brexit would do "catastrophic harm" to trust in the political process, Mrs May said: "We all have a duty to implement the result of the referendum."

Brexit: EU assurances fail to win back support from DUP