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By Paul Seddon & Rachel Russell
BBC News
Boris Johnson has said he will vote against Rishi Sunak's new Brexit deal for Northern Ireland at a Commons debate later.
The former PM said the deal would either mean Northern Ireland remained tied to the EU, or the UK would not be able to take advantage of Brexit.
The agreement, announced last month, rewrites the deal Mr Johnson struck with the EU in 2019.
Other Brexit-backing Tories are also expected to vote against the deal.
However, the scale of the rebellion is not yet clear, with a key group of Eurosceptic Tories undecided on how they will vote.
The vote is on a key part of the deal, known as the Stormont brake, that aims to give a future Northern Ireland Assembly a greater say on how EU laws apply to Northern Ireland.
It is also likely to be the only vote MPs get on Mr Sunak's overall Windsor Framework - which he agreed with European Commission President Ursula Von Der Leyen last month to replace the Northern Ireland Protocol Bill.
Under the Northern Ireland Protocol, inspection and document checks are carried out at Northern Ireland's ports instead of taking place at the Irish border - including goods travelling from Great Britain (England, Scotland and Wales) to Northern Ireland.
The checks apply even if the goods are due to remain in Northern Ireland.
Mr Sunak's new deal is aimed at significantly reducing the number of checks.
The prime minister said the Windsor Framework was proof that the UK has "taken back control", with his spokesperson adding it was "the best deal for the people and businesses of Northern Ireland".
In a statement, Mr Johnson said he found the proposed arrangements of the deal "unacceptable" as the best way for the UK to take back control would instead be to proceed with the NI Protocol Bill - which he previously agreed with the EU and it became part of international law and came into force on 1 January 2021.
"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," he said.
"That is not acceptable. I will be voting against the proposed arrangements today."
Leader Sir Jeffrey Donaldson said he will continue to work with the government on "outstanding issues" - even though Downing Street said there are no plans for any substantial change to the deal.
The vote will take place during Mr Johnson's testimony at the Privileges Committee on Wednesday, where he will be quizzed by MPs over partygate - a day after he admitted misleading Parliament by accident.
There will be a short break so MPs can vote.