Tory Eurosceptics have panned Rishi Sunak’s Brexit deal ahead of a crunch vote on Wednesday, warning that its central plank is “likely to be useless in practice”.

The European Research Group (ERG) delivered a withering assessment of the agreement struck with the EU, saying it fails to restore Northern Ireland’s place in the union.

In a damning dossier, it attacked Number 10 for making “misleading” claims about how the pact will remove swathes of EU law from the province and ditch border checks.

But despite the criticism Mark Francois, the ERG chairman, suggested the 30-strong group may not take a joint decision to oppose the deal in the Commons.

He said its members will meet on Wednesday morning to “discuss what attitude, if any, to take”, adding: “We don’t want to be hoist by our own petard.”

Asked whether its MPs may abstain, he said: “The group hasn’t taken a decision yet, and ultimately it will be down to every individual colleague in the group.”

Under the Northern Ireland Protocol, which was signed in 2020, Northern Ireland remained in the EU single market to avoid the need for a hard border with the Republic.

Instead, checks on goods were moved into the Irish Sea, creating new red tape barriers between businesses in Great Britain and the province.

Negotiations to fix problems created by the sea border began in 2021, but stalled until Mr Sunak became Prime Minister and improved relations with the EU. Last month, he signed the Windsor Framework with Ursula von der Leyen, the EU Commission president, which he said would restore Northern Ireland’s place in the union.

But the ERG’s legal assessment of the pact, drawn up by a “star chamber” of lawyers and MPs, has concluded that it falls well short of what he promised.

His central claim that the agreement fundamentally rewrites the protocol is “not correct”, according to the report, which says the original deal “remains intact”.

It found that under the deal “EU law will still be supreme in Northern Ireland” and will come “automatically under the jurisdiction” of the European Court of Justice.

Meanwhile, the rights of people in the province under the 1800 Act of Union, which ensured its equal treatment in trade matters, “are not restored by the agreement”.

The “star chamber” attacked the “green lane”, under which goods will be able to travel checks free from Great Britain to Northern Ireland, as “misleadingly characterised”, adding: “The green lane is only available for a limited range of goods considered not at risk and involves burdensome pre-registration and administration.”

It was critical of the “Stormont brake” mechanism, which the Government says will give Belfast powers to block new EU laws from applying in Northern Ireland.

The report says the mechanism has so many conditions attached to its use that it “is of very narrow application in theory and is likely to be useless in practice”. It describes the brake as a “highly restricted version” of a veto power available to Norway. Oslo has only tried to use that once in 30 years, and eventually gave up.

Under the Northern Irish version, new EU laws can be blocked from applying in the province if 30 Stormont Assembly members from two parties or more object. But the decision on whether to wield the veto will rest with Number 10, which can choose not to do so if no new regulatory barriers between GB and NI will be created.

In the event that the UK did pull the brake, Brussels would be able to dispute its use at an arbitration panel, which could overturn the decision and apply the rejected law.

Even if the arbitration panel ruled in Britain’s favour, the EU would still be able to take limited trade counter-measures to address any impact on European businesses. As a result the deal “risks incentivising future governments to copy future EU rules... so as to avoid the imposition of new checks across the Irish Sea,” the ERG warned.

It said the pact also creates “an incredibly dangerous precedent of allowing the EU to make regulations which apply only within the territory of Northern Ireland”.

The “star chamber” warned this “could be turned against the UK in future” and pointed out businesses in Great Britain risk being stung by Brussels’ state aid rules.

It also said the wins Mr Sunak has achieved are “vulnerable to suspension by the EU on grounds of suspected fraud, or termination by the EU on diversion of trade grounds”.

Downing Street has insisted that the agreement is “the best deal for Northern Ireland” and delivers the improvements asked for by businesses and people in the province. It said there are no plans to change the pact after the DUP announced that its MPs will vote against it in the Commons on Wednesday.

Sammy Wilson, the Unionist party’s chief whip in Westminster, said: “We didn’t get the outcome, so we have to continue the fight and we will continue the fight.”

Labour has said it will support the deal, so there is no chance of it failing to pass, but Number 10 will be determined not to have to rely on opposition votes. Mr Sunak has a working majority of 66, meaning he needs to contain any rebellion to fewer than 34 of his own backbenchers to avoid such embarrassment.

Speaking ahead of the publication of the report, Downing Street said the deal was a “significant” upgrade on the existing protocol.

Mr Sunak’s spokesman said: “It is not just us who has said it goes far beyond what has been secured before. Other groups, and indeed the DUP themselves, have said that significant progress had been made.”

He added that the Government had been open about what it had achieved in the negotiations and “the solutions the framework comes up with”, adding:“We have been as transparent as possible about the fact there is still an element of EU law, albeit much reduced, that applies in Northern Ireland, and about why that needs to happen.”