Rishi Sunak and Ursula von der Leyen are set to sign off on a new post-Brexit deal for Northern Ireland as Conservative Eurosceptics warn the “devil is in the detail”.

The Prime Minister and the European Commission President will hold “final talks” in Windsor, Berkshire, today before she will go on to meet The King.

But even if long-awaited changes to the Northern Ireland Protocol are agreed, it is likely to be only the start of the most challenging week of Mr. Sunak’s premiership.

He wants to win the support of the Democratic Unionist Party (DUP) so they agree to restore powersharing in Northern Ireland to get Stormont back up and running.

Conservative Brexiteer Jacob Rees-Mogg put Mr. Sunak on notice of a possible Tory revolt if the DUP does not support the deal, despite major concessions expected from the EU.

The former Cabinet minister told GB News: “It will all depend on the DUP. If the DUP are against it, I think there will be quite a significant number of Conservatives who are unhappy.”

He said that the position of Boris Johnson, who he described as the “biggest figure in UK politics”, will be “fundamental”.

“So, if the DUP doesn’t think that it meets its test, that will be very influential among Conservative MPs,” Mr. Rees-Mogg added to ITV’s Good Morning Britain.

“I’m afraid with all the EU deals, the devil is in the detail, so when people say ‘We need to see the legal text’, they are not larking about - they really want to see it to understand what the effect is.”

Mr. Rees-Mogg had said it would be a mistake for Ms von der Leyen to meet The King before MPs approve the deal, following anger at suggestions it could have been called the “Windsor Agreement”.

She will receive an audience with His Majesty at Windsor Castle despite concerns that the move will drag the monarchy into a political announcement.

The Prime Minister will speak to the EU chief at lunchtime before briefing his top ministers in a virtual meeting of the Cabinet.

Mr. Sunak and Ms von der Leyen will then set out the deal to voters in a joint press conference before the Prime Minister makes a statement to MPs in the Commons.

Tory Brexiteers on the European Research Group (ERG) will convene MP Sir Bill Cash’s so-called “star chamber” of lawyers to scrutinise the deal before deciding whether to back it.

The Northern Ireland Protocol, signed by Mr. Johnson as Prime Minister in 2020, was designed to prevent a hard border with the Republic of Ireland after Brexit.

Northern Ireland continues to follow EU rules on goods to prevent checks being needed when crossing into the Republic.

But the trade barriers created between Northern Ireland and Great Britain has angered Unionists.

The DUP collapsed powersharing in Stormont last year in protest at its impact, leaving Northern Ireland without an Executive or an Assembly.

DUP leader Sir Jeffrey Donaldson has issued seven tests that Mr. Sunak’s new pact will have to meet in order to win the party’s backing.

Chief among them is addressing what he calls the “democratic deficit” of Northern Ireland being subject to EU rules while not having a say on them.

No. 10 officials said Mr. Sunak would be entering into the talks with Ms von der Leyen looking to “return sovereignty to the people of Northern Ireland”.

“The Prime Minister wants to ensure any deal fixes the practical problems on the ground, ensures trade flows freely within the whole of the UK, safeguards Northern Ireland’s place in our Union, and returns sovereignty to the people of Northern Ireland,” Downing Street officials said.

Ms von der Leyen had been due to travel to Britain on Saturday to hold talks with Mr. Sunak, as well as reportedly meet His Majesty at Windsor Castle for tea, but the plans were scrapped.

The deal is expected to include check-free lanes for goods coming from Great Britain into Northern Ireland.

Deputy Prime Minister Dominic Raab said the reduction in trade red tape would lead to a “substantial scaling back” of the role of the ECJ.

But he refused on Sunday to rule out the court having a say on future legal cases, which could prove to be a flashpoint for DUP resistance.

If Mr. Sunak does allow a Commons vote, he is likely to win because Labour has agreed to support it, but he would want to win without relying on Opposition votes.