THERESA May has insisted that she would “never agree” to a Brexit deal, which trapped the UK in a permanent backstop arrangement.

Downing Street made the declaration after it was suggested that several Cabinet ministers, following a briefing with the Prime Minister, were alarmed by the prospect of a proposal to keep all of the UK in the EU customs union until a trade deal was signed, which they fear would in fact go beyond the end of the transition period and in effect become a permanent position.

But a No 10 spokeswoman said: "The Prime Minister would never agree to a deal which would trap the UK in a backstop permanently."

She explained that Mrs May stood by the terms of her June proposal for a backstop arrangement for the Irish border, to take effect only if a broader trade assessment had not been finalised.

The June document proposed that the whole of the UK would remain within a customs union until a deal on the future economic relationship was in place but made clear that Britain expected this would happen by the end of December 2021.

The spokeswoman added: "Our position is that this future economic relationship needs to be in place by the end of December 2021 at the latest."

On Thursday, David Mundell stressed how the backstop plan was only a fall-back guarantee should a trade deal not be completed by the end of the transition period and that the hope was it would never have to be implemented.

But he made clear that if it were enacted, it had to be time-limited; a proposal which Brussels has rebuffed, insisting any backstop has to be a permanent arrangement.

Mr Mundell said: “What is important is to recognise what we are talking about is a backstop provision. We are looking to have the future trade agreement and relationship set out almost simultaneously…Within days of the withdrawal agreement, we will hopefully be setting out the terms of our future relationship and we want those to come into play on January 1 2021 so we don’t want the backstop ever to be in place.”

He went on: “The backstop was there on the basis if you weren’t to get the other agreement implemented and, therefore, it’s still the case all the focus has to be on the initial agreement being implemented.

“It is fair, if there is to be a backstop, to set a time-limit on that backstop so that it couldn’t possibly exist either when the principal agreement came into full force or was indefinite so as to allow a situation, which wasn’t intended to be the negotiated arrangement for the UK and for Northern Ireland’s relationship with the EU to become a permanent fixture,” said the Secretary of State.

Mr Mundell also insisted reports that the PM was willing to see Northern Ireland remain in the European single market as the rest of the UK left it were untrue. “There is no plan for Northern Ireland to be in the single market,” he declared.

The Scottish Secretary stressed how he and Ruth Davidson, the Scottish Conservative leader, had made clear to Mrs May there fundamental opposition to any Brexit carve-out for Northern Ireland as it would undermine the integrity of the United Kingdom. Mr Mundell signalled that if that were ever part of any proposed deal it would be a resignation matter for him.

Meanwhile after Northern Ireland’s Democratic Unionists threatened to vote down the Autumn Budget if any Brexit deal politically separated the region off from mainland Britain, Damian Green, the former Cabinet Office Minister, stressed how Mrs May was "a very Unionist Prime Minister".

He claimed the PM and the DUP, whose 10 MPs prop up the Tory Government, were "as one" in their desire to see Northern Ireland treated like the rest of the UK.

Mr Green told BBC Radio 4's Today: "She is a very Unionist PM, we are the Conservative and Unionist Party.

"She and the DUP are as one in wanting to make sure than Northern Ireland isn't in some way treated differently from the rest of the United Kingdom in trade or customs terms and that is an absolutely key point in these negotiations, as the DUP have pointed out."