One of the architects of the Good Friday Agreement has warned that Theresa May’s failure to deal with problems posed by Northern Ireland’s border threatens to bring Brexit negotiations “crashing down”.

Writing exclusively for The Independent, Jonathan Powell accused the Prime Minister of committing “the worst possible sin” of having “boxed herself in”.

The ex-chief negotiator in Belfast peace talks said Ms May’s approach will create a hard border between Northern Ireland and the Republic, laying the seed for dangerous “identity politics” that once fostered division and hatred there.

In a warning to Jeremy Corbyn, he also said Labour’s plan of “a customs union” similarly failed to negate the need for a hard border – which he said could be removed only if the UK stays fully aligned with the single market.

On Monday, the EU and UK provisionally agreed how the Brexit transition period will operate, but still failed to settle how to handle Northern Ireland’s border.

In his article for The Independent, Mr Powell argued that Ms May’s recent Mansion House speech “has not solved the substantive problems”.

He went on: “Indeed, her problems on Brexit may only just have begun and it may turn out that the insoluble problem of the Northern Ireland border is the issue that finally brings the entire negotiation crashing down.”

Jean Claude Juncker calls for more clarity from Theresa May over Brexit

Ms May has promised Remain-backing Tory MPs and the Irish government that there will be no hard border between Northern Ireland and the Republic, while also promising Brexiteer MPs the UK will leave the EU’s customs union.

But EU negotiators say the positions are irreconcilable – that the only way to ensure no border is for the UK to remain aligned with the customs union and rules of the single market.

Theresa May has therefore committed the worst possible sin a negotiator can commit. She has boxed herself in. She may hope that she can again fudge the issue, by pretending the border is soft rather than hard. But the fudge doesn’t work anymore. Ex-chief negotiator in Northern Ireland Jonathan Powell

Mr Powell said the Prime Minister had managed to “fudge” the issue up to now, but that a negotiating strategy that saw her rule out options more than a year ago has forced her into a corner.

The chief of staff to Tony Blair from 1997 to 2007 went on: “Theresa May has therefore committed the worst possible sin a negotiator can commit. She has boxed herself in.

“She may hope that she can again fudge the issue, by pretending the border is soft rather than hard. But the fudge doesn’t work anymore.

“The Government will have to agree the legal text of a divorce treaty in October if we are to leave by March next year. And legal texts are not susceptible to constructive ambiguity.”

Donald Tusk on Brexit negotiations: Ireland first

He points out that Labour’s solution of “a customs union” also does not solve the problem, given the draft withdrawal treaty sees the EU calling for calling for Northern Ireland to remain aligned not just with the customs union, but all rules of the bloc’s “internal market”.

Mr Powell began work as a British diplomat in 1979, working through tense, ill-tempered but ultimately successful negotiations that led to the return of Hong Kong to China.

He argues that the British Government’s insistence on having a free trade deal allowing it to choose areas of the single market to opt into, without taking on the obligations, is at the heart of the problem.

Any trading relationship with Europe on that basis will involve a hard border, he said, and with that a potential return to troubles of the past.

He writes: “The Good Friday Agreement was all about identity. People in Northern Ireland could feel British, Irish or both because there is no visible border.

“Once we again block off the small back roads with huge concrete slabs to stop smuggling and put in checkpoints on the main roads we reopen the issue of identity.

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The prime minister did not appear in parliament to see another defeat PA Brexit so far: in pictures Labour and Conservative MPs resign and create the Independent Group Back row of Chris Leslie, Gavin Shuker, Chuka Umunna and Mike Gapes, middle row of Angela Smith, Luciana Berger and Ann Coffey and front row of Sarah Wollaston, Heidi Allen, Anna Soubry and Joan Ryan PA Brexit so far: in pictures Non-biding votes on amendments to Brexit motion On February 27 he house held a series of votes, unanimously calling for the UK and EU to guarantee citizens rights in a no-deal scenario AFP/Getty Brexit so far: in pictures Attorney General publishes legal advice A hammer blow for May as Geoffrey Cox said her renegotiated deal can still leave UK in backstop against its will. Mr Cox did say the prime minister’s efforts had reduced the risk of the UK being trapped in the backstop indefinitely. 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If MPs rejected it for a third time, the EU said Britain must propose a new plan by April 12. Ms May said she will not support a long delay because it would mean Britain participating in elections for the European Parliament Getty Brexit so far: in pictures Brussels confirms preparations for a no-deal Brexit are completed They warned that it is “increasingly likely” the UK will crash out. In a statement the European Commission (EC) said “preparedness and contingency work”, which the EC has been conducting since December 2017, was now finished. The announcement came days after EU leaders agreed to a request by Theresa May to extend the UK’s Brexit date AFP Brexit so far: in pictures May resigns British Prime Minister Theresa May reacts as she delivers a statement in London, Britain, May 24, 2019. REUTERS/Simon Dawson SIMON DAWSON Reuters

“That does not mean we are automatically tipped back into the Troubles again ... but it does mean we force Northern Ireland back into identity politics.”

Mr Powell backed findings of the recent report from the Commons Northern Ireland Affairs Committee, which concluded ministerial claims that a hard border could be avoided using technology were “blue sky thinking”.

He predicted a solid attempt to resolve the issue would be kicked back from this week’s European Council, something borne out by announcements on Monday.

The former diplomat now anticipates that by October Ms May will be forced to accept either a customs border in the Irish Sea, thus losing the support of DUP MPs propping up her administration, or keeping the whole UK in the single market and customs union, potentially sparking a Brexiteer rebellion.

He said: “[European Council President] Donald Tusk reiterated in Dublin that it would be ‘Ireland First’. It would be a colossal gamble for the Government to stake everything on being able to break that united front at the last moment.

“And if they don’t, we risk crashing out of the EU without an agreement at all.”

Theresa May sets out trade plans for new relationship with the EU post-Brexit

Speaking in Brussels on Monday, Brexit Secretary David Davis said it remains the UK’s intention to achieve a “partnership that is so close as to not require specific measures in relation to Northern Ireland”.

He added: “We have also reached consensus on the full set of issues which need to be addressed in any solution in order to avoid a hard border, which is why, last week, we set out a work programme to tackle them.

“There are also some elements of the draft protocol – such as the Common Travel Area – on which we agree.