A key group of Brexit-supporting Conservative MPs are yet to say whether they will vote for Rishi Sunak’s Brexit deal.

Parliament will vote tomorrow on the so-called Stormont brake – a key part of the prime minister’s Windsor Framework agreement that would allow the Northern Ireland Assembly to reject new incoming EU rules.

European Research Group (ERG) chairman Mark Francois refused to confirm how the group’s members will vote, despite the group’s lawyers branding the brake “practically useless”.

The DUP have said it wants the brake to go further and grant Stormont the power to reject or repeal old conditions imposed by the bloc on Northern Ireland, not just new future EU rules.

Francois said ERG members would discuss the matter again on Wednesday ahead of the vote.

Francois said his lawyers had found that “EU law will still be supreme in Northern Ireland” and that the “rights of its people under the 1800 Act of Union are not restored”.

He told journalists: “The DUP have made their position very plain – they are going to vote against.

“We, to some degree, have been critical of the government for not allowing people enough time to digest everything.

“Because we need to allow people time to digest this, the ERG will meet again tomorrow… once people have had an opportunity to digest all this documentation.

We as a group will discuss what attitude, if any, to take and we will be having that meeting just before PMQs.”

Downing Street rejected the ERG’s concerns with the Brexit deal.

Prime Minister Rishi Sunak’s spokesman said: “The brake addresses the democratic deficit and provides a clear democratic safeguard for the people of Northern Ireland.

“I think we continue to strongly believe that this is the best deal for Northern Ireland, both for the people who live there, businesses who seek to operate there and it is to the benefit of the entire United Kingdom.

“This is a good deal and we continue to urge all parliamentarians to back it.”

The ERG and DUP’s opposition, however, will not be enough to block the deal as Labour has already said it will vote it through.