The EU’s chief Brexit negotiator has ruled out ever accepting Theresa May’s Brexit plan B, in a major blow to the prime minister’s bid to get MPs to back her plan.

Michel Barnier said repeated requests for a time limit on the controversial backstop had already been discussed and rejected twice by EU leaders.

But he also signalled there could be a way to avoid a hard border in Ireland in the event of a no deal, telling an EU committee on Wednesday: “We will have to find an operational way of carrying out checks and controls without putting back in place a border.”

In a separate joint interview with continental newspapers Mr Barnier said “we cannot tie the backstop to a time limit” as suggested by the prime minister.

He said the withdrawal agreement on the table was “the only possible option” for Britain and also ruled out the possibility of a so-called “managed no deal” as advocated by some Tory Brexiteers.

“In the case of no deal, action will of course be taken to ensure that planes can land but … the ‘no deal’ cannot be a sum of mini-deals and be a situation of ‘business as usual’,” he told Le Monde, Rzeczpospolita and Luxemburger Wort.

“Even an agreement for an ordered Brexit will cause disruptions and have serious consequences. The ‘no deal’ even more so.”

At the committee the chief negotiator elaborated, warning that the only way to avoid the UK crashing was to endorse another option on the table.

“There appears to be a majority in the Commons to oppose a no deal but opposing a no deal will not stop a no deal from happening at the end of March”, he told the committee. “To stop no deal, a positive majority for another solution will need to emerge.”

Following the defeat of her withdrawal agreement by the largest margin in the history of parliament, Ms May said she would try and secure a time limit on the controversial backstop policy.

But Mr Barnier said: “The question of limiting the backstop in time has already been discussed twice by the European leaders, in November and in December 2018.

“This backstop is the only one possible because an assurance is no longer operational if it is for a limited time. Imagine if it were to be limited in time and the problem arose after expiry: it is useless!”

He was backed up later on Wednesday by Frans Timmermans, the first vice president of the European Commission.

“We will support Ireland … the backstop is a red line we cannot negotiate with the British government,” Mr Timmermans told an event in Krakow, Poland.

In the weeks since the defeat of the withdrawal agreement the EU has focused on encouraging the PM to revisit her red lines, and signalled its willingness to re-work the so-called “political declaration” that lays out the future trading relationship.

Labour and other opposition parties back a much closer economic relationship with the EU, floating possibilities like common single market standards and a customs union.

Mr Barnier suggested the question of the backstop would “become relative” if the future relationship was revisited – signalling that it might be a way out of the current impasse.

“If the British government wishes to revisit the future relationship and be more ambitious, then it would be possible to agree on the global package and the question of the ‘backstop’ would become relative,” he said.

“If I understand the British debates right; there is a desire to find a way. But if the government and the MPs don’t move their lines, we are going inevitably into ‘no deal’.”

The prime minister has repeatedly ruled out shifting her red lines on issues like a customs union and free movement.

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The political backdrop to Mr Barnier’s comments is increasingly complicated. The Commons is considering a new amendment to rule out a no-deal Brexit – forcing the government to seek an extension or revocation of Article 50 if the UK gets too close to the deadline without an agreement.

But in Ireland, the government is facing a political backlash over suggestions a hard border would return if Britain crashes out. Leo Varadkar’s cabinet was put in a tight spot earlier in the week after a Commission spokesperson said there would “obviously” be new infrastructure in the event the withdrawal agreement was not signed. Though Mr Barnier appeared to endorse this suggestion in ensuing interviews, by Wednesday afternoon he had apparently softened his tone by suggesting there could be another way around the problem.

That concession itself, however, is likely to fuel opposition to the withdrawal agreement in Westminster – where Eurosceptic MPs are likely to interepret Mr Barnier’s latest comments as an admission that the backstop, which they fiercely oppose, is not necessary. They may also be emboldened by a suggestion by the Polish foreign minister earlier in the week that the EU should accept a time limit on the backstop, as the MPs demand.