Theresa May is under pressure to publish secret legal advice that is believed to state that parliament could still stop Brexit before the end of March 2019 if MPs judge that a change of mind is in the national interest. The move comes as concern grows that exit talks with Brussels are heading for disaster.

The calls for the prime minister to reveal advice from the country’s top legal experts follow government statements declaring that Brexit is now unstoppable, and that MPs will have to choose between whatever deal is on offer next year – even if it is a bad one – or no deal at all.

Disquiet has been growing among pro-remain MPs, and within the legal profession and business community, about what is becoming known as the government’s “kamikaze” approach. Ministers insist that stopping Brexit is not an option, as the British people made their decision in last year’s referendum, and the article 50 process is now under way, however damaging the consequences might turn out to be when negotiations are concluded.

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Worry about lack of flexibility has intensified following the chaotic Conservative party conference in Manchester, and May’s ill-fated speech. European leaders now doubt whether she has the political authority to move negotiations forward towards a satisfactory deal.

The prominent lawyer Jessica Simor QC, from Matrix chambers, has written to May asking her to release the legal advice under the Freedom of Information Act. Simor says she has been told by “two good sources” that the prime minister has been advised “that the article 50 notification can be withdrawn by the UK at any time before 29 March 2019 resulting in the UK remaining in the EU on its current favourable terms.

“Such advice would also accord with the view of Lord Kerr, who was involved in drafting article 50, of Jean-Claude Piris, former director general of the EU council’s legal service, and of Martin Selmayr, a lawyer and head of cabinet to the European commission president.” She says “there is no time to waste” and adds: “It is important that this advice is made available to the British public and their representatives in parliament as soon as possible.”

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Leading figures who oppose a hard Brexit, and reject the idea that the process is unstoppable, argue that the public is being hoodwinked by ministers into believing that there is now no alternative to leaving. Simor says this is a “policy” judgment, not a legal one.

Nick Clegg, the former deputy prime minister, who publishes a book this week on how to stop Brexit, told the Observer: “The claim that article 50 is irreversible was always a myth put about by Brexiters who want to stop the British people from changing their minds. Theresa May’s threat that MPs will have to vote for whatever deal she presents to them next autumn, otherwise the UK will crash out of the EU without a deal, is also patent nonsense. Article 50 was never the one-way conveyor belt to Brexit as claimed by the government. It can be stopped at any point.”

He said that MPs should ask themselves one question when they come to vote in a year’s time: “Does the deal measure up to the promises made by Brexiters to their constituents before the referendum? If not, MPs should reject the deal, urge the government and the EU to stop the clock, and give the country the opportunity to think again.”

Facebook Twitter Pinterest Nick Clegg claims that the threat that the UK could crash out of the EU without a deal was ‘patent nonsense’. Photograph: Chris Williamson

Clegg added: “As countless EU leaders have said in private and in public – most recently [French] President Macron – there remains a route back for the UK into a reformed EU. This does not mean simply turning the clock back to the day before the referendum, but forging a new status for the UK in an outer circle of EU membership as the core countries proceed with deeper integration.”

Clegg suggests that former prime minister John Major and current Netherlands prime minister Mark Rutte should co-chair a special UK-EU convention that would have the task of “repositioning Britain in one of the outer rings of the EU’s orbit”.

Labour MP Chuka Umunna said that, as it becomes clearer that the Brexiter pledges would not be met, the public had a right to know that leaving was not the only option. “Brexit is proving hugely more complex than people were told it would be. Many of the promises made – like the famous £350m extra per week for the NHS – clearly won’t be delivered. As a result, increasingly voters ask whether, if a good agreement cannot be reached and the final deal looks fundamentally different to that which they were promised, can the UK decide to halt the article 50 process? The consensus among lawyers is that we still have a choice.

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“Legally, there is nothing inevitable about this process and the UK retains the right to change its mind. Politically, many EU partners have indicated they have no objections to this.”

There is also growing anger over the government’s continuing refusal to publish studies it is conducting on the impact that Brexit will have on different sectors of the economy. Labour MP David Lammy said: “It is frankly quite absurd that the Department for Exiting the European Union is refusing to publish these studies while ministers go around talking about the repatriation of parliamentary sovereignty and taking back control.”

A DExEU spokesperson said: “We made our position clear in the supreme court. As a matter of firm policy, our notification will not be withdrawn. The British people voted to leave the EU and we will deliver on their instruction. There can be no attempts to remain inside the EU and no attempt to rejoin it.”