Theresa May is scrambling to contain a Tory mutiny over what looks to be the final sticking point in Brexit divorce talks today.

The Prime Minister faces Cabinet unrest and a fresh onslaught from Boris Johnson amid claims she is on the brink of striking a settlement with Brussels.

The package could involve the whole UK staying in a customs union with the EU as a ‘backstop’ to avoid a hard Irish border plus extra regulatory checks in the Irish sea – something that could cause fury from Mrs May’s DUP allies.

Downing Street has dismissed rumours the arrangement could last indefinitely, and tried to play down reports a deal is effectively in place.

But Brexit Secretary Dominic Raab is understood to be demanding that the UK is able to leave unilaterally within three to six months if the backstop does kick in.

The mechanism for ending a backstop is the last significant sticking point in the divorce talks, MailOnline has been told.

The focus of the tensions was laid bare today when Irish deputy PM Simon Coveney insisted that there cannot be a ‘time limit’ on any backstop, and the UK must not be able to end it unilaterally. His message was retweeted by EU negotiator Sabine Weyand as Brussels ratcheted up pressure on Mrs May.

However, any more concessions on the issue could spark a major Cabinet showdown tomorrow. The PM is expected to present a new policy paper on Brexit to ministers, with some Eurosceptics fearing an ‘ambush’.

Former foreign secretary Boris Johnson has criticised Prime Minister Theresa May after rumours of a secret Brexit deal were leaked

Prime Minister Theresa May

Former foreign secretary Boris Johnson has criticised Prime Minister Theresa May after rumours of a secret Brexit deal were leaked

The continuing tensions were laid bare today when Irish deputy PM Simon Coveney insisted that there cannot be a 'time limit' on any backstop, and the UK must not be able to end it unilaterally

The continuing tensions were laid bare today when Irish deputy PM Simon Coveney insisted that there cannot be a ‘time limit’ on any backstop, and the UK must not be able to end it unilaterally

In an article for the Sun today, Mr Johnson said the plan was an ‘absolute stinker’ and a ‘Christmas present of the finest old Brussels fudge’.

He warned the government was proposing to ‘hand over an amazing £40billion for absolutely nothing in return’.

‘We have agreed to remain in vassalage forever,’ he said.

‘Even after we leave — according to this so-called deal — we will remain in a nonsensical ‘implementation period’ in which we will be effectively nonvoting members of the EU.

‘For the first time in a thousand years we will have to accept foreign made laws — with no power to change or make those laws. We will be a vassal state — a colony — for at least 18 months, probably more.’

He added: ‘If the customs union whispers are true, then Mrs May will be leading us to little more than Brexit in name only’. 

What is the final issue in the Brexit talks?

Theresa May insists the Brexit deal is 95 per cent done – but that the final issue of the Irish border backstop may be the hardest part.

The backstop is about what will happen to the Irish border if the Brexit transition ends before a final UK-EU trade deal is in place.

Transition is currently due to end in December 2020.  

The EU is still insisting that in the absence of a full trade deal, Northern Ireland should stay in the EU customs union while the rest of the UK leaves to ensure the Irish border remains open.

Britain says the whole UK should stay in customs rules for a temporary period so there is no border in the Irish Sea. 

The EU says it is not opposed to this in principle but had said there was no time this autumn.

Brussels now appears to be prepared to do a UK-wide backstop in the divorce deal – but it is not clear whether this will convince it to drop its own backstop.

Mr Johnson called on the Cabinet to turn its back on the plans ahead a crunch meeting tomorrow morning. 

There have been mounting signs of optimism that the UK and EU can thrash out a divorce package. 

Many of the thorniest issues on the Irish border backstop – which would come into effect if no other way is found to avoid a hard line between Northern Ireland and the Republic – appear to have been resolved.

However, Mr Raab is said to have passed on a tough message to Mr Coveney last week, saying the UK must be able to bring the backstop to an end unilaterally and with three to six months’ notice.

Cabinet Office minister David Lidington was apparently dispatched to Dublin to smooth over the issue on Friday.  

But the clash could yet torpedo the whole process.

Mr Coveney tweeted this morning: ‘The Irish position remains consistent and v clear⁩ that a “time-limited backstop” or a backstop that could be ended by UK unilaterally would never be agreed to by IRE or EU. 

‘These ideas are not backstops at all + don’t deliver on previous UK commitments.’

The message was endorsed by Ms Weyand, Michel Barnier’s deputy, who complained it was ‘still necessary to repeat this’.  

As Tory Brexiteers become increasingly anxious about the shape of Mrs May’s plan, Mr Johnson slammed the EU over the Irish border issue and accused Brussels of creating an issue that shouldn’t exist.

According to Mr Johnson, the EU Commission doesn’t want a hard border and neither does anyone else involved and the situation was ‘designed by the EU’ to stop any future independence bids.

He added: ‘Brexit was meant to be about taking back control. Under this plan – we surrender control to Brussels.’

The Prime Minister has had a difficult week and her former foreign secretary is convinced that she will come under more fire when MPs realise what the deal will really involved.

He branded the rumoured plan ‘a national humiliation’ and called for it to be thrown out.  

In response, a Downing Street spokesman said: ‘We are not staying in the customs union — the Prime Minister could not have been clearer.

‘We are making good progress on the future relationship and 95 per cent of the Withdrawal Agreement is now settled. Negotiations are ongoing.’

Referring to reports of the agreement, she added: ‘This is categorically not the deal.’ 

Mrs May is facing a Cabinet showdown this week over new Brexit concessions designed to secure a deal this month.

The Prime Minister will brief her senior ministers tomorrow on new proposals to break the Brexit deadlock, including plans designed to satisfy EU demands for additional checks on goods travelling between Northern Ireland and the rest of the UK.

Brexiteer Cabinet ministers are demanding legal advice from the Attorney General Geoffrey Cox before signing off any new proposals, amid fears they are set to be bounced into accepting a deal ‘at any price’.

He labelled the PM's supposed deal as  being 'appalling' and described it as 'Christmas present of the finest old Brussels fudge'

He labelled the PM’s supposed deal as  being ‘appalling’ and described it as ‘Christmas present of the finest old Brussels fudge’

In a sign of the Cabinet’s twitchiness, Mrs May yesterday held a series of conference calls with ministers to reassure them she had not agreed a Brexit deal without their knowledge.

Downing Street dismissed reports that Mrs May has sealed a ‘secret Brexit deal’.

But Cabinet ministers complain they are being ‘kept in the dark’ about the true state of the negotiations and fear a deal could be sprung on them as early as this week, with no time to consider the long-term implications.

What is the Brexit transition and how could it be extended?

What is the Brexit transition period?

The EU and UK agreed in March that there would be a 21-month transition after Brexit day on March 29, 2019. It is due to expire on December 31, 2020.

In the period Britain will still obey all EU rules without having any say in setting them, pay membership fees, and have the same trade terms.

The current plan has not yet been voted on in Parliament.

How long could it be extended?

There is a new proposal to extend transition for another few months, possibly to the end of 2021.

Why is there talk of extending it?

March’s agreement also included a commitment to create a ‘backstop’ on the Irish border – a back up plan to ensure the border stays open in the absence of a long term trade deal.

There is a deep dispute over how this should work so the new suggestion is a longer transition could allow a full trade deal to be completed in time for the backstop never to be used.

The theory is this takes pressure off the exact form of the backstop, making it easier to agree and solving the last issue in the divorce deal.

Does anyone support extending it?

The EU says it is willing to adopt a longer transition if Britain asks for it.

Theresa May says Britain is not asking for a longer transition – but might accept a mechanism for extending the current transition.

Tory Brexiteers hate the idea, fearing it will just be extended again and again and leave Britain facing billions in extra charges.

Government sources tried to calm the row today by insisting an extension would only be acceptable if the EU accepted the UK’s version of the backstop.

Would it solve the Irish problem?

Nobody knows. Both sides are still far apart on how the backstop will work and there is doubt a full trade deal can be finished even by the end of 2021.

What happens if transition is extended?

Britain would continue to follow all EU rules without having a say on how new ones are made – meaning continued free movement, European Court judgements and bills to Brussels.

Trade across the Channel would also continue under today’s rules.

One Cabinet source said Mrs May appeared to be ‘desperate’ to agree a deal this month to allow more time to get it through Parliament and avoid having to implement costly contingency plans for a no deal scenario.

‘The fear is that in her desperation to get it done this month she will be willing to give up anything,’ the source said.

‘Everyone wants a deal, but they don’t want it at any price. That’s why half the Cabinet wants to see legal advice from Geoffrey Cox before signing anything off. But when are they going to tell him what they are proposing? Are we just going to be presented with a fait accompli?’

David Davis demanded legal advice is made public ahead of a parliamentary vote on the final deal, warning that the deal was ‘one of the most fundamental decisions that a government will have taken in modern times’.

He added: ‘The authority of our constitution is on the line. So we have to get this right and the Government has to be transparent.’

Government sources were tight-lipped about the contents of the ‘update on negotiations’ Mrs May will provide to the Cabinet tomorrow.

But it is understood she has now tabled new proposals for regulatory checks on goods travelling to and from Northern Ireland. No 10 dismissed as ‘speculation’ reports that the checks could be carried out by British officials away from the border.

Mrs May has rejected Brussels demands for Northern Ireland to remain on its own in the customs union if the deal on the economy fails to secure frictionless trade.Instead she has proposed a UK-wide ‘backstop’ which could see the whole country remain temporarily in the customs union while an alternative solution is found.

Mrs May is seeking an ‘exit clause’ to allay fears this could leave the UK trapped permanently inside the customs union.

But ministers want cast-iron assurances that the UK alone will have the final say over its departure from any ‘temporary’ arrangement.

The EU’s chief Brexit negotiator Michel Barnier is said to be softening his opposition to a UK-wide backstop. Government sources insist Mrs May is genuine in her desire to extricate Britain from the customs union.

International Trade Secretary Liam Fox is said to have come to her aid, telling a recent Cabinet meeting of the vital need to secure a deal with the EU.

A source said the prominent Brexiteer told ministers: ‘It’s amazing that some people think it’s fine to have a World Trade Organisation relationship with the EU but are desperate to have trade deals with the rest of the world.’

Meanwhile, Germany’s ambassador to the UK became the latest senior figure to talk up a Brexit deal. Peter Wittig told the BBC’s Andrew Marr show: ‘There is a political will to come to a negotiated settlement.’