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.
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.
He said: “There cannot be a temporality clause, as it is impossible for there to be a unilateral withdrawal from it. The backstop must be and remain credible.”
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies