The joint letter says the deal on offer is "a fair compromise"
The European Commission has published a letter, giving assurances to Britain over Brexit.
The letter, from European Council Donald Tusk and EU Commission President Jean-Claude Juncker, is addressed to British Prime Minister Theresa May.
It insists there can be no renegotiation of the backstop, which would be used in the event of a no-deal scenario to prevent a hard border in Ireland.
On the Brexit deal, it says: "It represents a fair compromise and aims to ensure an orderly withdrawal of the United Kingdom from the European Union, thereby limiting the negative consequences of Brexit.
"That is also why we wish to establish as close as possible a relationship with the United Kingdom in the future, building on the political declaration, which the leaders of the 27 EU member states agreed with you.
"It is also why we want negotiations to this effect to start as soon as possible after the withdrawal of the United Kingdom from the European Union."
On the backstop issue, it says: "he Commission can confirm that, just like the United Kingdom, the European Union does not wish to see the backstop enter into force.
"Were it to do so, it would represent a suboptimal trading arrangement for both sides.
"The Commission can also confirm the European Union's determination to replace the backstop solution on Northern Ireland by a subsequent agreement that would ensure the absence of a hard border on the island of Ireland on a permanent footing."
The letter also clarifies: "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the backstop to enter into force in whole or in part, it is intended to apply only temporarily, unless and until it is superseded by a subsequent agreement."
It comes as Mrs May made a speech, in which she said Britain is now more likely to stay in the European Union than to leave without a deal.
Addressing workers at a factory in the English city of Stoke-on-Trent, where 69.4% of voters backed leave the bloc, Mrs May said that if the referendum result is not honoured, people's faith in politics will suffer "catastrophic harm".
When the people are asked to decide something, their response carries a "profound significance", she added.
Speaking the day before UK MPs vote on her withdrawal agreement, she claimed that some in Westminster will use "every device available to them" to "delay or even stop Brexit".
It was reported on Sunday that there is a backbench plot to wrest control of the legislative agenda away from Mrs May if her deal fails.
Were such a plan to succeed, the British government would lose control of parliamentary business, threatening its ability to govern, putting Brexit legislation at risk.
Mrs May now believes, Downing Street said, that "based on the evidence of the last week", MPs managing to block Brexit is more likely than the UK leaving the EU without a deal.
The UK is due to leave the EU by default on March 29th.
Article 50 - the mechanism for member states to quit the bloc - was triggered on that day two years ago by Mrs May, starting the countdown to Brexit.
She has repeatedly told MPs that "no deal is better than a bad deal".
Speaking a few weeks after calling off a previous vote on her withdrawal agreement because she knew it would be rejected, Mrs May will ask MPs to consider various scenarios.
"Imagine," she said, "if an anti-devolution House of Commons had said to the people of Scotland or Wales that despite voting in favour of a devolved legislature, parliament knew better and would over-rule them. Or else force them to vote again."
She added: "What if we found ourselves in a situation where parliament tried to take the UK out of the EU in opposition to a remain vote?
"People's faith in the democratic process and their politicians would suffer catastrophic harm."
Mrs May said previous referendums have been honoured: "When the people of Wales voted by a margin of 0.3%, on a turnout of just over 50%, to endorse the creation of the Welsh Assembly, that result was accepted by both sides and the popular legitimacy of that institution has never seriously been questioned.
"Parliament understood this fact when it voted overwhelmingly to trigger Article 50. And both major parties did so too when they stood on election manifestos in 2017 that pledged to honour the result of the referendum."
At least 90 Conservative MPs are expected to vote against the Brexit deal.
Additional reporting: IRN