Brussels has rejected Theresa May’s new customs proposal less than 24 hours after the prime minister set it out in a bid to placate Brexiteers in her cabinet.

European Commission officials told The Independent Ms May’s plan would be unacceptable and would go back on previous commitments made by British negotiators.

A day earlier the prime minister had said the “backstop” plan to avoid a hard border in Northern Ireland – which keeps Britain in alignment with the single market and customs union if no other agreement is reached – would be time limited.

The move was an attempt to assuage Brexiteers such as Boris Johnson, who fear that it would become a backdoor way to keep Britain tied indefinitely to the EU through the customs union and single market.

The controversial fallback arrangements look increasingly likely to come into play, with no other plan for the Northern Ireland border in sight and Ms May’s cabinet deadlocked on what Britain’s future customs relationship with the EU should be.

European Commission officials close to the talks told The Independent that British negotiators had already made written commitments for the backstop to apply “unless and until” another solution was found in Northern Ireland, and that there was no way it could be time limited.

Facing a backlash over the plan from her pro-Brexit ministers, the prime minister sought to calm their fears, telling reporters on Monday: “If it is necessary, it will be in a very limited set of circumstances for a limited time.”

But one Brussels source said: “It will apply for as long as there is no credible alternative. It can’t be time limited or it’s not a backstop.”

Commission officials have pointed to the first page of the draft withdrawal agreement, which includes a note, signed off by both sides, stating that the backstop alignment can be ended only if another solution is found.

“The negotiators agree that a legally operative version of the ‘backstop’ solution for the border between Northern Ireland and Ireland, in line with paragraph 49 of the joint report, should be agreed as part of the legal text of the withdrawal agreement, to apply unless and until another solution is found,” a note on page 1 of the withdrawal agreement says.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

A UK government spokesperson said: “The prime minister set out her position yesterday. The fallback solution put forward by the EU is not acceptable, and that’s why we will be bringing forward our own proposal. We want to make progress as soon as possible.”

Cabinet ministers including Boris Johnson and Michael Gove have signalled their unease at the backstop plan in recent days. The foreign secretary issued an apparent veiled warning to the PM while speaking to reporters on a trip to Latin America, warning her against “betrayal”.

“Brexiteers fearing betrayal over the customs backstop must understand that the prime minister has been very clear that neither option is an outcome we desire – we want a deal with the EU and she will deliver it,” he said.

“I’m convinced that the prime minister will be true to her promises of a Brexit deal that sees Britain come out of the customs union and single market, have borders as frictionless as possible, reject European Court of Justice interference, control immigration and free to conduct unhindered free trade deals across the world.”

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Speaking separately, Mr Gove was at pains to make clear that the backstop was “intended not to be implemented” and was “there just in case”.

Brexiteer cabinet ministers have repeatedly rolled over for Downing Street after issuing similar veiled warnings on issues ranging from the Brexit divorce bill to the transition period, however.

The row over the backstop is the latest twist in the wrangling over what post-Brexit customs arrangement the UK and EU will have.

The backstop was agreed on paper in December as part of a deal – the joint report – struck between Theresa May and Jean-Claude Juncker in Brussels. The section on the backstop says that “in the absence of agreed solutions” to the Northern Ireland border, “the United Kingdom will maintain full alignment with those rules of the internal market and the customs union which, now or in the future, support North-South cooperation, the all-island economy and the protection of the 1998 Agreement”.

It has since become clear that the UK and EU have begun to interpret the agreement in different ways, however. Downing Street says the entire UK will stay in alignment under the backstop, while Brussels says the agreement was intended only to maintain full alignment in Northern Ireland.

The difference in interpretation comes because the UK desperately also wants to prevent customs checks at Irish Sea ports between Great Britain and Northern Ireland. The DUP, on whose votes the Conservatives rely for a majority in the House of Commons, are staunchly unionist and have threatened to block any plan that puts any checks between Northern Ireland and Great Britain. Ms May has said “no British prime minister” could agree to such a state of affairs.

Brussels is, however, concerned that the UK would gain access to the single market without having to commit to its four freedoms – effectively gaining a preferential deal.