Theresa May will have to delay Brexit even if she gets her exit plan through parliament this month, Michel Barnier has said.

The EU’s chief negotiator said there was now not enough time for the European Union to ratify the withdrawal agreement, even if MPs back the plan in a vote expected to be held on 12 March.

In a joint interview with several European newspapers, Mr Barnier also said the EU was willing to provide further assurances that the backstop will be temporary.

Ms May has promised that MPs will be allowed to vote on extending Article 50 if they decide to reject her deal a second time, but Mr Barnier said an extension would be needed even if the agreement gains parliament’s approval.

His comments will come as a blow to the prime minister, who has insisted Britain will be able to leave as planned on 29 March if MPs back her deal.

Top politicians' Brexit tweets haven't aged well Show all 18 1 /18 Top politicians' Brexit tweets haven't aged well Top politicians' Brexit tweets haven't aged well These billboards were plastered by campaign group Led By Donkeys @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well Undated handout photo issued by Led by Donkeys of their billboard featuring a blank tweet in Jeremy Corbyn's Islington North constituency in protest over what they claim is his lack of leadership on Brexit. PRESS ASSOCIATION Photo. Issue date: Tuesday February 12, 2019. For the past month, campaign group Led by Donkeys has been putting up posters all over the country featuring quotes mostly from hard-Brexit MPs which claimed the process of leaving the EU would be easy. The Labour leader is the group's latest target because of his failure to back a second referendum on Brexit. See PA story PROTEST Donkeys. Photo credit should read: Led by Donkeys /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 Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter Top politicians' Brexit tweets haven't aged well @ByDonkeys / Twitter

On whether it would be possible for the EU to ratify the agreement if the UK parliament signs it off on 12 March, Mr Barnier told papers including Germany’s Die Welt and Spain’s El Mundo: “No, no. There would have to be an extension that would be called ‘motivated’ or ‘technical’.

“But you have to ask the UK. If there is a vote on 12 [March] and it takes two months for the procedure, it would be justified.”

Asked if an extension to Article 50 was the only option if a Brexit deal is to be finalised, he answered: “Yes, you could say that.”

Mr Barnier said the EU would most likely agree to such an extension, but that member states would need to be convinced there was a good reason for it.

He said: “The hypothetical duration would depend on what they want it for. I know there is the idea of ​​an extension without conditions, but you have to know that an extension would serve to fix a problem, not to delay the solution.

“In my opinion, do not postpone the problem for three or six months, do not risk prolonging the general uncertainty of Europe beyond the elections, but it will be for the heads of state to respond.”

Ms May’s hopes of securing the Commons’ backing for her deal rest on obtaining fresh concessions from the EU on the backstop – the main source of opposition from Tory Brexiteers.

Stephen Barclay, the Brexit secretary, and Geoffrey Cox, the attorney general, have been locked in talks with Brussels in a bid to secure legally binding guarantees that the backstop, which would see the UK enter into a customs union with the EU if a trade deal cannot be agreed, would be temporary.

Mr Barnier said Brussels was open to giving further assurances that this is the case.

He said: “We know that there are misgivings in Britain that the backstop could keep Britain forever connected to the EU.

“This is not the case. And we are ready to give further guarantees, assurances and clarifications that the backstop should only be temporary.”

He announced that the EU is considering providing the new guarantees in a joint interpretative document – an add-on to the withdrawal agreement – saying this could enshrine assurances made in a letter from Donald Tusk, president of the European Council, and Jean-Claude Juncker, president of the European Commission, in January.

Mr Barnier said: “ Guarantees, assurances, can be given in a document, perhaps next to the treaty, not in the political declaration.

“We can imagine an interpretative document, where there are elements such as those already in the letter from Tusk and Juncker. That was a letter from two presidents of two institutions. If the document is agreed with the United Kingdom, it would be much stronger.”

However, he insisted that any new concessions will not include either a time limit on the backstop or a unilateral exit mechanism for the UK – a key demand of many Tory Eurosceptics.