In essence, the backstop deal would be an agreement between the UK and the EU over the customs union.
The deal would help to avoid a hard border in Ireland if no agreement was made in Brexit negotiations – but will also keep the UK tied to the Brussels bloc after the Brexit end date.
Theresa May is battling for Government unity on her backstop proposals for the Irish border as the Brexit war Cabinet meets.
The Prime Minister is facing a tough response from Brexiteers over plans to allow a time-limited deal, which could come into force in the event no agreement was reached on withdrawal.
No 10 says it does not believe the contingency plan will ever come into effect because a customs arrangement would be agreed as part of the overall withdrawal deal.
A customs deal is seen as vital to preventing a hard border between Northern Ireland and the Republic of Ireland.
Downing Street will outline plans for a “temporary customs arrangement” with the EU should no permanent solution be agreed before the UK’s 2019 exit.
Full details of the government’s Brexit “backstop” proposal are to be outlined today – but BBC political editor Laura Kuenssberg said it might not specify an end date.
Cabinet tensions on the backstop initiative emerged after Mr Davis was believed to be insisting that the UK should be able to unilaterally withdraw from any border agreement in order to maintain leverage with Brussels in ongoing negotiations.
When pressed on whether he could remain in post if the backstop deal did not meet his full approval, Mr Davis said: “That’s a question I think for the Prime Minister to be honest.”
Brexit backstop: David Davis and Theresa May could row over the plan
Former Brexit minister David Jones said signing a unilateral deal with the European Union to avoid a hard border in Ireland could keep Britain shackled to the bloc.
Speaking to BBC Radio 4’s Today programme, Mr Jones said: “I think if we were to find ourselves in a position whereby we are in this customs arrangement indefinitely with only the promise that we would be leaving as soon as possible it would not be acceptable.
“The other thing that we have to remember is that, as I understand it, Britain will not unilaterally be able, once it signed up to this arrangement, to withdraw from it.
“It would be the Hotel California scenario: we’d have checked out but we wouldn’t have left.”
Tory Brexiteer Henry Smith also warned the so-called “backstop” customs system for Northern Ireland is “unworkable”.
Brexit: Theresa May will release her Brexit backstop plans
He said: “I think backstop doesn’t provide an ultimate end date and, therefore, doesn’t give the assurance that people need that we are cleanly leaving the European Union.
“I think tying the whole United Kingdom into the inertia of staying in a lot of those EU single market and customs union rules would be something that actually wouldn’t work for the sort of Brexit that the Government and indeed the Prime Minister in her Florence and her Lancaster House speech has said that she wants.”
But not everyone is so pessimistic about the plans.
Mike Cherry, Federation of Small Businesses (FSB) National Chairman, said: “Proposing a time-limited UK wide ‘Brexit backstop’, where we stay part of a customs arrangement after the agreed transition period, is a common sense solution that buys the Government more time to secure a pro-business Brexit deal.
“Momentum around both the negotiations with the EU 27 and the passage of the Withdrawal Bill has clearly been lost, while the clock continues to tick closer to the March 2019 deadline.
“A further safety net for small businesses should give them more time to adapt and respond to any changes that will come with the UK’s exit from the European Union and will avoid any hardening of the border on the island of Ireland. This is crucial for our small businesses in Northern Ireland.
Brexit news: David Davis is expecting a time limit on the backstop
“A ‘backstop’ cannot be seen as a full-stop to negotiations. Smaller firms need certainty and understanding of what post-Brexit landscape is being shaped.
“Businesses need to know how they will be able to trade, hire new staff and what sort of regulatory environment they will be working in.
“These businesses need to see a strong urgency and willingness from all parties to swiftly move forward with negotiations. If progress remains slow, there is a real fear that growth, job creation and productivity will be irreparably damaged.”
Will David Davis resign over Brexit backstop plans?
A senior Tory Brexiteer warned Theresa May she cannot afford to lose Brexit Secretary David Davis in a Cabinet row over future customs arrangements with the EU.
Mr Davis is reported to have considered resigning over the so-called “backstop” plan to avoid a hard border between Northern Ireland and the Republic if there is no Brexit deal.
Davis is understood to be deeply unhappy about the lack of a firm end date in the agreed text – and now rumours are swirling around Westminster that he could quit.
The Brexit Secretary said after a speech in Westminster: “On time limits, I think the Prime Minister has already made public the fact that we expect to put a time limit on the backstop proposal.”
Mr Jones, who said the backstop would keep Britain tied to the EU despite the vote to leave, also told the BBC Radio 4 Today programme: “We need to make sure that David Davis stays at the negotiating table.
“Anything that caused him to leave would be deeply regrettable and deeply damaging to the country.”
Mr Jones said the proposal as it stood would not be acceptable to the “mass” of the Conservative Party.