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Ireland’s prime minister openly attacked Boris Johnson today as a growing impasse over Northern Ireland threatened to derail Brexit talks.

Leo Varadkar tore into the Foreign Secretary’s claim that London’s congestion charge showed that technology could avoid a hard border between Northern Ireland and the Republic.

“It’s really up to those who can’t accept the backstop to actually come up with those alternative solutions,” Mr Varadkar said. He added scathingly: “It can’t be theoretical stuff about congestion charges.”

Amid rising tensions, a Brexit-backing former Cabinet minister, David Jones, accused the EU of trying to “annex” Northern Ireland.

A former senior European Commissioner, Karel De Gucht, said the entire Brexit negotiations were heading “at a fast pace... towards a cliff edge”.

The border issue was the biggest blockage expected in a 120-page legal text due out from the European Union today.

Mr Johnson was in hot water after a leaked letter revealed that he foresaw the border between Northern Ireland and Ireland could become “significantly” harder after Brexit, contrary to government assurances it would be frictionless. Fellow senior ministers distanced themselves from his letter, sent to Theresa May two weeks ago. One told the Standard: “It is not Government policy.” Asked if it showed that a hard border was being contemplated, the minister said: “By the Foreign Secretary.”

Mr Varadkar caused consternation by suggesting that Northern Ireland could become part of “a common regulatory area” with full single market rules covering the island of Ireland, separate from the UK mainland.

“In the same way that we have a common travel area between Britain and Ireland we could have a common regulatory area involving Northern Ireland,” he claimed.

Mr Johnson hit back: “What is going on is that the issue of the Northern Irish border is being used quite a lot politically to try and keep the UK in the customs union — effectively the single market — so we cannot really leave the EU. That is what is going on.” The leak of Mr Johnson’s letter was seen by Brexiteers as an attempt to undermine the minister’s public case for “divergence” from EU regulation in the UK.

“It is a very positive letter,” Mr Johnson told Sky News after a morning jog. He hit out at the “inverted pyramid of objections” over the issue.

Former prime minister Sir John Major was set to intervene this afternoon in a speech. With the 30th anniversary of the Good Friday Agreement falling on April 10, he was expected to make the case for closer alignment.

Former Foreign Office mandarin Sir Simon Fraser warned that today’s text shifted negotiations onto a legal phase where the Government could no longer get by on creative ambiguity. “When the lawyers are in the room you have to have precision, you have to nail things down,” he said.

On the back of the leaked letter, Northern Ireland Secretary Karen Bradley said the Government remained committed to no new border infrastructure on the island of Ireland.

She also told the Northern Ireland Affairs Committee in Westminster: “The constitutional and economic integrity of the UK is paramount and something that the Government will ensure is what we will achieve.”