Boris Johnson will vote against Rishi Sunak’s Brexit deal on Wednesday in a major boost for Tory rebels who warn it is not the right solution for Northern Ireland, The Telegraph can reveal.

The former prime minister said in a statement to this newspaper that the proposals would keep the province “captured by the EU legal order” and were “not acceptable”.

Mr Johnson is expected to interrupt his appearance during the House of Commons Privileges Committee hearing on partygate to vote when the division bell rings.

Liz Truss, Priti Patel and Iain Duncan Smith have also all confirmed that they will also vote against the Brexit deal.

It means at least three former Tory leaders – Mr Johnson, Ms Truss and Mr Duncan Smith – will be opposing their successor’s plan.

Mr Johnson told The Telegraph: “The proposed arrangements would mean either that Northern Ireland remained captured by the EU legal order - and was increasingly divergent from the rest of the UK - or they would mean that the whole of the UK was unable properly to diverge and take advantage of Brexit.

“That is not acceptable. I will be voting against the proposed arrangements today. Instead, the best course of action is to proceed with the Northern Ireland Protocol Bill, and make sure that we take back control.”

The decision sees Mr Johnson reprise the role he adopted during the final year of Theresa May’s premiership, when he put himself at the front of a group of Tory MPs voting against her Brexit proposals.

It is a blow to Mr Sunak’s attempts to make sure that Conservative MPs pass his proposals and that he is not reliant on the support of opposition parties to secure the measure.

Some figures tapped into discussions among Eurosceptics believe that more Tory MPs than the 20 expected could vote down the proposal - though only a handful had gone public by Tuesday night.

The Commons vote on Wednesday afternoon is on the so-called ‘Stormont break’, a measure that would give the Northern Ireland assembly powers to reject new EU laws being imposed from Brussels.

The province has a more interconnected relationship with the EU since Brexit than the rest of the UK given agreements to keep the land border between Northern Ireland and Ireland open and free of customs checks.

However there are varying interpretations about how legally watertight the Stormont break, which emerged from a renegotiation over the wider Northern Ireland Protocol deal overseen by Rishi Sunak, is in reality.

The European Research Group (ERG), made up of Brexiteer Tory MPs, claimed on Tuesday that it was “likely to be useless in practice” and accused Downing Street of making “misleading” claims.

The Prime Minister is unlikely to face overall defeat in the Commons on the statutory instrument implementing the Stormont Break, which Number 10 has sought to portray as symbolising a wider all encompassing vote on Mr Sunak’s Brexit deal.

The Labour leadership, which is attempting to reassure voters that they would be trusted with Brexit if they won back power, has made a merit of promising to support the move even if scores of Tory MPs rebel on the matter.

Relying on the votes of opposition parties to pass the Brexit plan, which could happen if more than 30 Tory MPs vote it down, would cause political damage to Mr Sunak as it would question his authority and ability to secure Conservative support for key legal moves.

The vote is due to take place on Wednesday afternoon.