Leading Brexit supporter Jacob Rees-Mogg MP has warned Britain faces “perpetual purgatory” in the European Union, after the government signalled it would be willing to remain tethered to the bloc’s Customs Union after 2021.

Prime Minister Theresa May’s so-called ‘War Cabinet’, where disputes over the country’s EU negotiating positions are hashed out, has reportedly agreed continued Customs Union membership after the end of a mooted 2019-2021 ‘transition period’ after objections by Brexit supporters Michael Gove and Boris Johnson were voted down.

The measure is intended to act as a “backstop” if the EU refuses to come to a reasonable agreement over the customs border between the Republic of Ireland and the British province of Northern Ireland, where both sides have deemed the erection of physical customs barriers an unacceptable outcome.

“The risk of the Government using all its mental energy on the fallback position is that they create a position that is more attractive than a permanent deal,” warned the popular Somerset MP, who heads scores of Brexit-supporting Tory MPs who have combined under the European Research Group banner.

“We have gone from a clear endpoint, to an extension, to a proposed further extension with no endpoint. The horizon seems to be unreachable. The bottom of the rainbow seems to be unattainable. People voted to leave, they did not vote for a perpetual purgatory.”

Australia: Staying in EU Customs Union Would Turn Brexit Britain into ‘Irrelevant, Humiliated Dependency’ https://t.co/wKB7khh4Sc — Jack Montgomery ن (@JackBMontgomery) March 29, 2018

Rees-Mogg has always made a point of displaying his loyalty to the Prime Minister in public, but the two are reported to have clashed behind the scenes during recent efforts to bring Brexiteer backbenchers on-side at Downing Street.

Rees-Mogg is said to have told the Prime Minister that the government should walk away from the negotiations and accept a ‘no deal’ exit from the EU if Brussels will not be reasonable on the Irish border question.

Neither the United Kingdom nor the Republic of Ireland are in favour of erecting customs barriers, and Rees-Mogg believes the government should simply leave the border open and tell the EU to pay for controls if it insists on having them — but May is unwilling to take such a tough line, believing it could lead to a referendum on Northern Ireland leaving the UK.

Rees-Mogg believes the people of Northern Ireland would vote in favour of the British union if such a poll were held — as they did previously in 1973, and as the Scots did in 2014 — but the Prime Minister is not so confident.

“Mrs May said, ‘I would not be as confident as you. That’s not a risk I’m prepared to take. We cannot be confident on the politics of that situation, on how it plays out,’” a source told The Times.

“She slapped him down very hard,” claimed another MP.

Follow Jack Montgomery on Twitter: @JackBMontgomery