Under-pressure Theresa May has refused several times in parliament to say when a so-called Irish backstop arrangement could end in the event of a no-deal Brexit.
The prime minister told the Commons she would expect it to expire in December 2021 but added she does not want to see the backstop having to be used at all.
Many Conservative MPs – reportedly including some cabinet ministers – are concerned a “temporary” backstop would become permanent.
They want any agreement to include a firm date for it to lapse.
The biggest obstacle to a withdrawal agreement is the issue of the Irish border, with the UK and EU sides divided on the best way of avoiding the need for checkpoints.
Regulatory and customs checks at hundreds of crossing points between Northern Ireland and the Republic would cause massive disruption to individuals and businesses.
There are fears that checkpoints and cameras could become the target of sectarian violence.
The Democratic Unionist Party, which is propping up Mrs May’s government in Westminster, has made clear it will not back anything which means Northern Ireland being treated differently from the rest of the UK.
Several MPs from across the political spectrum, including ex-foreign secretary Boris Johnson as well as Labour’s Hilary Benn and Liz Kendall, questioned Mrs May in the Commons about when the backstop would end.
Mr Johnson said if December 2021 “was not the deadline then could she say when it is”.
The PM said she was working to ensure the backstop never comes into place.
Mr Benn said: “Since the expectation of an end date is not the same as a definite end date when is she going to tell her party that we cannot have a fixed artificial time limit on that fall back which the government is trying to negotiate with the EU?”
The PM went on to explain what the purpose of the backstop was, adding: “It is there for the time until the future relationship can come into place. We expect that to be no later than December 2021.”
Ms Kendall said: “Does the PM stand by the commitment the Brexit Secretary [Dominic Raab] made to the House last week that the government will publish a specific end date to the Irish backstop as part of the withdrawal agreement?”
The PM replied: “One of the issues we are discussing with the European Union remains this issue of ensuring that the backstop is a temporary arrangement and we cannot stay in a permanent relationship of that sort with the EU.
“The backstop is intended as an insurance policy for the people of Northern Ireland.”
The prime minister has urged the EU not to allow a stand-off over the backstop to derail the Brexit talks, saying she believed a deal was achievable.
She told MPs “real progress” had been made on the withdrawal agreement and the political declaration on future relations as the 29 March 2019 deadline looms when the UK will leave the bloc.
Mrs May said progress had been made on Northern Ireland but two problems remain – the EU’s demands over the backstop and the need for a time limit on the backstop.
The EU is insisting on a “backstop” arrangement as a fallback to be used until a broader deal is in place.
This would involve Northern Ireland effectively remaining part of the European customs area, with checks taking place at ports and ferries between the region and the British mainland instead of at the border.
But Mrs May has said no prime minister could accept it, as it would create a customs border down the Irish Sea and undermine the integrity of the UK.
She has put forward an alternative backstop, featuring a temporary arrangement keeping the whole of the UK in a customs union, which she expects to be lifted by the end of 2021.
But that would only be acceptable to Brussels if there is a “backstop to the backstop”, so that when the temporary UK-wide arrangement runs out, the Northern Irish carve-out would come into effect.
Jeremy Corbyn urged Mrs May to “put the country before her party” and stand up to the “reckless voices” on the Conservative benches.
The Labour leader said: “For too long this country has been held hostage to those in her party who want to drive through a race to the bottom Brexit deal that lowers rights and standards, sells off our national assets to the lowest bidder.
“It is clear that the prime minister’s failure to stand up to the warring factions of her own side have led to this impasse.”
European Council president Donald Tusk said EU leaders will discuss Brexit on Wednesday in Brussels where Mrs May will address them on the current state of negotiations.
He tweeted: “On a #Brexit deal: It always seems impossible until it’s done.”
EU’s Brexit co-ordinator Guy Verhofstadt said: “It is our very strong view that the withdrawal agreement must include a workable, legally operational and all-weather backstop for the Ireland/Northern Ireland border fully in line with the joint report of last December and the Prime Minister May’s letter to President Tusk of 19 March 2018.
“Without such a backstop, the European Parliament would not be in a position to give its consent to the withdrawal agreement.”