The European Research Group (ERG) of hardline Brexiteer Conservative MPs has rejected Rishi Sunak’s revised plan for trading arrangements in Northern Ireland, saying it would keep EU law as “supreme”.

Announcing the decision, Mark Francois, the Tory MP who chairs the group, said its study of the Windsor Framework also found that the apparent veto for Northern Irish politicians over new EU regulations was “practically useless”.

Speaking to reporters in parliament following an ERG meeting, Francois strongly indicated the group would recommend voting against the plan in the UK’s House of Commons on Wednesday, but said a final decision would be made at another meeting before the debate.

The damning verdict increases the potential size of a Tory rebellion against the plan, and follows the decision on Monday by the Democratic Unionist Party to also reject the deal.

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A leading DUP MP, Sammy Wilson, said on Tuesday that his party would “continue the fight” against the plan, also indicating that it could continue its boycott of the Northern Ireland Assembly at Stormont.

Speaking on BBC’s Good Morning Ulster on Tuesday Wilson, the DUP’s Brexit spokesperson, was asked by Colum Eastwood, the leader of the moderate nationalist SDLP, when the DUP could return to Stormont.

Wilson replied: “Colum, you may be prepared to roll over, to having powers taken away from the people who are elected to Stormont, we’re not.”

The ERG remains influential, if not to the same extent as during the Brexit negotiations under Theresa May, and it is not clear how much its view will influence the vote. It does not publish any statistics about its membership.

The deal was unveiled last month by Sunak after talks with Ursula von der Leyen, the European Commission president. The vote on Wednesday will be on one part of the plan, the so-called Stormont brake veto. It will be decided as a statutory instrument, a piece of secondary legislation, rather than a bill or a formal change to an existing law.

The ERG has produced a 49-page report by two lawyers affiliated with the group, part of what it styles its “star chamber”, published on Tuesday.

Summarising its findings, Francois said they had concluded the Stormont brake was not effective, and that the check-free “green lane” for the bulk of goods moved between Northern Ireland and the rest of the UK would not work as billed.

Francois said: “The star chamber’s principal findings are: that EU law will still be supreme in Northern Ireland; the rights of its people under the 1800 Act of Union are not restored; the green lane is not really a green lane at all; the Stormont brake is practically useless; and the framework itself has no exit, other than through a highly complex legal process.”

Asked if this meant the ERG would recommend MPs vote against the plan, Francois said this would be decided at a meeting on Wednesday, and that so far the group had not decided “what attitude, if any, to take”.

He added: “This is by no means just for the ERG. We would like all members of parliament, ideally, to read this before the vote tomorrow, so they know exactly what they’re voting on.”

The ERG’s report sets out the view that the revised protocol does not remove any EU laws from Northern Ireland, trade restrictions will still exist in goods to and from the rest of the UK, and that the Stormont brake is so unwieldy as to be virtually pointless.

Asked if the ERG felt it had been misled by ministerial assurances about the plan, Francois replied: “I’m going to leave that to others to judge.” Questioned about whether a no vote would be inevitable given the scale of the group’s worries, he said only: “Let’s wait and see what happens tomorrow.”

[ Windsor Framework: what is the Stormont Brake in the Brexit deal for Northern Ireland? ]

Asked about the ERG’s verdict, Sunak’s official spokesperson rejected the idea that the Stormont brake was useless, calling it “a significant step change to what had previously been agreed”. He added: “It fundamentally restores or deals with the democratic deficit that existed under the protocol or beyond.”