A SENIOR No10 official is accused of branding the Brexit Secretary “a f***ing nightmare”.

The extraordinary attack on Dominic Raab was alleged to have come during a drink-fuelled dinner with Brexiteer MPs on Monday night.

Brexit Secretary Dominic Raab is said to have been branded a 'f***ing nightmare' by a senior No10 official

Some of the PM’s senior aides suspect Mr Raab of “freelancing” during his weekly talks with the EU’s chief negotiator Michel Barnier by going further than No10’s tight script.

They also blame Mr Raab for a fresh tactic being deployed by the Brussels chief to impose the EU’s long standing demand for a customs border for goods down the Irish Sea.

The Sun can also reveal that Mr Barnier wants to police a new trade border between Northern Ireland and Great Britain with technology to make it as invisible as possible.

He is billing the tactic – known as Maximum Facilitation, and pinched from Brexiteers - as a new solution to the bitter impasse over how to keep the Irish border open.

Some of the PM’s senior aides suspect Mr Raab of 'freelancing' during weekly talks with EU chief negotiator Michel Barnier by going further than No10’s tight script

Mr Raab was also said to have angered No10 by sending Mr Barnier into a rage recently by challenging the EU to impose a hard border in Ireland if there is no deal, as the UK would refuse.

Two of the nine backbench Tory MPs at the boozy No10 dinner have told The Sun that the very senior official told them that Mr Raab “has been a fucking nightmare ever since he arrived”.

The rising star was promoted into the key Cabinet job in July after David Davis walked out.

One of the MPs in the room said: “It was an utterly extraordinary thing for such a senior aide to the Prime Minister to say about a Cabinet minister. We were stunned.

“The tension No10 is under trying to make the disastrous Chequers plan work is clearly getting to some of them.”

The PM is fighting to keep the Chequers plan alive Credit: EPA

The senior official last night adamantly denied making the comment.

But he did admit using sarcasm during the dinner that “could have been wilfully misunderstood”.

A No10 spokesman said: “It is 100% nonsense and did not happen”.

Sources close to Mr Raab insisted it was “utter rubbish” that he is conspiring with Mr Barnier for a customs border down the Irish Sea, or that he has gone rogue during talks.

Mr Raab and Mrs May are in lockstep over her red line not to see the UK divided up in any way by the Brexit deal, the source added.

It emerged yesterday that British negotiators have handed Mr Barnier new data about internal UK trade, on his request.

The figures give a detailed breakdown of the volume and type of trade in goods that takes place between Great Britain and Northern Ireland.

Ireland’s ambassador to the EU, Declan Kelleher, said Mr Barnier wants the data to show his version of the backstop won’t create an Irish Sea border.

Mr Raab is said to have angered No10 by sending Mr Barnier into a rage recently by challenging the EU to impose a hard border in Ireland if there is no deal, as the UK would refuse

He said: “The backstop in no way changes the constitutional status of Northern Ireland within the UK. It’s not a border down the Irish Sea.

“Essentially it would involve addressing issues such as technical controls at the ports between Northern Ireland and Great Britain which already exist.

“Nothing revolutionary is being proposed here, it’s simply a question of building on what’s already been achieved.”

An EU source told The Sun that officials would look at it with a view to seeing whether checks could be moved away from the border or aided by technology.

They said: “We can’t de-dramatise it by ourselves. We’re still open to the UK coming with something but as time goes by it looks less likely.”

Dominic Raab was promoted into the key Cabinet job in July after David Davis walked out Credit: Getty Images - Getty

A second source added: “The UK has provided us with some of the data, which we’ll look at together with them.

“This data will be helpful to better understand trade flows between Northern Ireland and Great Britain.”

British negotiators have reportedly given their EU counterparts assurances that they will compromise on the backstop after the Tory party conference.

But EU officials are still wary of the fact that any compromise on the border could be torpedoed by eurosceptics.

A diplomatic source said: “The real question is what will happen with the domestic dynamics in the UK.

“The Tory conference is a huge political risk factor to the final stage of the negotiations.”

Dominic Raab says Brexit talks could collapse if the EU doesn’t match our ambition and pragmatism

Mr Raab and Mr Barnier held their weekly talks over the phone yesterday after plans for a face to face meeting were cancelled.

The Brexit Secretary said: “While there remain some substantive differences we need to resolve, it is clear our teams are closing in on workable solutions to the outstanding issues in the Withdrawal Agreement, and are having productive discussions in the right spirit on the future relationship”.