A deal between Britain and the EU on the Irish border is not “close” despite hopes of a breakthrough, the EU’s chief Brexit negotiator has warned.

Michel Barnier said there was “still a real point of divergence” on the Northern Ireland issue, after Theresa May called for a “review” mechanism to be attached to the EU’s planned backstop that would guarantee no hard border.

The warning came as Theresa May told her Cabinet that a deal ”would not be done at any cost”.

The Northern Ireland border issue is the main final stumbling block to a withdrawal agreement – though the delay resolving it also mean negotiators have not worked out even an outline of a trade deal either.

Ireland’s government on Monday publicly rejected a proposal by Brexit Secretary Dominic Raab for Britain to be able to unilaterally pull out of the Irish ‘backstop’ regime after three months, suggesting such a policy would not be “worth the paper it was printed on”.

Mr Barnier told Belgian broadcaster RTBF on Tuesday: “For now, we are still negotiating and I am not, as I am speaking to you this morning, able to tell you that we are close to reaching an agreement, since there is still a real point of divergence on the way of guaranteeing peace in Ireland, that there are no borders in Ireland, while protecting the integrity of the single market.”

The delay, which has is pushing talks even further beyond the original October deadline, means it is increasingly unlikely a special Brexit summit will be held in November to finalise a deal. The EU has said December is too late to reach an agreement, though some in Brussels suspect the deadline could slip yet further.

Tory eurosceptics, including in Theresa May’s own Cabinet, are concerned that plans to keep the UK inside a customs union with the EU temporarily to help prevent a hard border could see Britain never fully leave the bloc’s orbit.

The UK had itself requested that the so-called “backstop” apply to the whole UK rather than just Northern Ireland, as originally proposed by the EU – in order to avoid border checks between Northern Ireland and Great Britain. But Brexiteers are now worried about the implications of the policy, and want some kind of time-limit. Mr Barnier has said a time-limited backstop would be “no backstop at all”.

A Downing Street spokesperson said on Tuesday that Cabinet ministers needed more time to consider ways to ensure Britain could not be bound to the EU by the backstop indefinitely.

The spokesperson indicated that a meeting of the body on Tuesday had discussed such options, but that ministers failed to reach a final conclusion.

A customs union would still not prevent all border checks, the EU has said, because of the continued need for health and regulatory checks on animal products. Michel Barnier has said these would increase ten-fold after exit between Great Britain and Ireland, if the UK leaves the single market.

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

Giving an account of the Cabinet meeting, the spokesperson added: “The Prime Minister said she was confident of reaching a deal. She said that, while the UK should aim to secure a withdrawal agreement as soon as possible, this would not be done at any cost.

“The Prime Minister said that, once agreement was reached on a withdrawal agreement, it remains the case that nothing is agreed until everything is agreed and it will be subject to securing an acceptable full future framework.”