There is “no room whatsoever” to renegotiate the Brexit deal, the president of the European Commission has said, ahead of Theresa May’s trip to Brussels to seek concessions.

Speaking in the European Parliament on Tuesday Jean-Claude Juncker said re-opening the withdrawal agreement “will not happen”.

He said the best the prime minister could hope for was “further clarity and further interpretations without reopening the withdrawal agreement” when she meets EU leaders in Brussels this week.

The Ms May is desperate to gain concessions from the EU on the deal she struck last month after it was given an overwhelmingly hostile reception by MPs.

She postponed a planned vote on the agreement after it became clear that it would be rejected by the House of Commons, with her own MPs opposed to its “backstop” clause that would keep the UK in a customs union with the EU.

But Mr Juncker said: “The backstop is necessary for Ireland. Ireland will never be left alone.”

The Commission president described Brexit as “a surprise guest” at the European Council summit scheduled for this end of this week, adding: “I am surprised because we had reached an agreement on 25 November together with the government of the UK.”

“The deal we have achieved is the best deal possible, it is the only deal possible, and so there is no room whatsoever for renegotiation,” he said.

“There is room enough to give further clarity and further interpretations without reopening the withdrawal agreement.

“This will not happen. Everyone has to know that the withdrawal agreement will not be re-opened.”

Last night European Council president Donald Tusk, who speaks for member state governments collectively, issued a similar warning. He said: “We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification.”

Member states, too, have taken the same line. Arriving in Brussels for a meeting on Tuesday, France’s Europe minister Nathalie Loiseau told reporters: “We are very much concerned about the postponement of the vote on the withdrawal agreement.

“We’ve done a lot to help the UK, this withdrawal agreement is the only possible agreement, and we’ve done a lot of concessions to reach it. We sincerely hope that there can be a majority to ratify the withdrawal agreement but we have to stand ready for a no-deal and we are preparing for it.”

French government 'concerned' at cancellation of brexit vote

Leo Varadkar, the Irish prime minister, said on Monday it was "not possible to reopen any aspect" of the agreement "without reopening all aspects", adding that it was "the only agreement on the table". He said no clarification of the deal could contradict its contents.

The promise of "clarification" is unlikely to assuage Conservative eurosceptic rebels, who have said they will not vote for the plan unless the backstop is removed, given a time-limit, or an exit clause. The EU has ruled out these ideas, saying the backstop would no longer be a backstop if it was not "all-weather".

Theresa May has admitted her deal would not pass parliament and has gone to seek concessions (Parliament TV. ) (Parliament TV)

It is understood that EU member states could stretch to a so-called "side-declaration" at the summit to assist Ms May. This non-legally binding political declaration would simply spell out how both sides expect the deal to be interpreted, though it would not change its contents.

The prime minister’s chief Brexit negotiator Olly Robbins returned to Brussels on Monday and was spotted in the European Commission headquarters – apparently trying to seek concessions on the deal.

Cabinet ministers resign over Brexit deal Show all 13 1 /13 Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Esther McVey Britain's Work and Pensions Secretary Esther McVey arrives to attend the weekly meeting of the cabinet at 10 Downing Street in London. - Britain's Work and Pensions Secretary Esther McVey resigned from the cabinet over draft Brexit deal AFP/Getty Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @EstherMcVey1 of the resignation letter addressed to Prime Minister Theresa May from Works and Pensions Secretary Esther McVey. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms McVey has resigned, saying the Brexit deal âdoes not honour the result of the referendumâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Dominic Raab British Brexit Secretary Dominic Raab give a press conference at the end of the final round of talks in Brexit negotiations at the European Commission in Brussels, Belgium EPA Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @DominicRaab of the resignation letter addressed to Prime Minister Theresa May from Brexit Secretary Dominic Raab. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Mr Raab has resigned, saying he âcannot in good conscience support the terms proposed for our deal with the EUâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Rehman Chrishti Vice Chairman of the Conservative Party Rehman Chrishti tendered his resignation letter this afternoon PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Anne-Marie Trevelyan Parliamentary private secretary in the Department of Education Anne-Marie Trevelyan resigned stating that she cannot support the deal Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @annietrev of the resignation letter addressed to Prime Minister Theresa May from Anne-Marie Trevelyan, a parliamentary private secretary in the Department for Education. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms Trevelyan has resigned saying she cannot support the Brexit deal after negotiations âbuilt on the UK trying to appease the EUâ. See PA story POLITICS Brexit. Photo credit should read: @annietrev/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Sam Gyimah Universities minister Sam Gyimah resigned on November 30, claiming the government's decision to pull out of the EU's Galileo satellite navigation system as a deciding factor PA Cabinet ministers resign over Brexit deal Shailesh Vara Shailesh Vara who has quit as Minister of State for Northern Ireland, saying he cannot support Theresa May's Brexit agreement, which he said "leaves the UK in a halfway house with no time limit on when we will finally be a sovereign nation" PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Suella Braverman Brexit minister Suella Braverman has resigned, stating “It is not what the British people, or my constituents, voted for in 2016.” Cabinet ministers resign over Brexit deal Ranil Jayawardena Parliamentary private secretary to the ministry of justice Ranil Jayawardena resigned as he could not back the deal "in good conscience"

Ms May herself is due to visit Brussels on Tuesday for meetings with Mr Juncker and Mr Tusk; she will return again for the European Council summit with leaders on Thursday and Friday, where Brexit will be on the agenda.

On Monday a European Commission spokesperson said: “We have an agreement on the table that was endorsed by the European Council in its Article 50 format on the 25 November. As president Juncker said, this deal is the best and only deal possible.