The DUP says its MPs will vote against a key aspect of Rishi Sunak’s Brexit deal when it is put to a crunch vote in parliament this week.

In a statement, party leader Sir Jeffrey Donaldson said while the Windsor Framework represented “significant progress” in addressing concerns with the Northern Ireland Protocol, it does not deal with some of the “fundamental problems at the heart of our current difficulties”.

As a result, they have said they will vote against the first aspect of the deal to be considered by MPs, the Stormont brake.

This would allow a minority of politicians in Belfast to formally flag concerns about the imposition of new EU laws in Northern Ireland – a move that could see the UK Government veto their introduction in the region.

Downing Street said this would address the so-called “democratic deficit” caused by the protocol, but Sir Jeffrey said the brake “is not designed for, and therefore cannot apply, to the EU law which is already in place and for which no consent has been given for its application”.

“Whilst representing real progress, the ‘brake’ does not deal with the fundamental issue which is the imposition of EU law by the protocol,” he said.

The DUP’s opposition does not mean the legislation will fail, but it makes the likelihood of the unionist party rejoining powersharing slimmer.

The unionist party pulled out of the arrangement for devolved government in Northern Ireland early last year in protest at the protocol.

The mechanism was agreed by Boris Johnson under his “oven ready” Brexit deal to prevent a hardening of the land border on the island of Ireland – which all sides agreed was necessary to preserve peace.

But it led to trade barriers being created between Great Britain and NI – effectively creating a customs border down the Irish sea.

Having spent weeks scrutinising Mr Sunak’s deal to replace the protocol, the DUP released a statement earlier on Monday saying “there is more work to do” to tackle issues that led to the collapse of powersharing in Stormont.

And senior DUP MP Ian Paisley said he would “categorically” vote against the government, saying the framework “did not cut the mustard” when it came to passing the party’s seven tests.

It could spell trouble for Mr Sunak, with many Tory Eurosceptics waiting to see what the DUP says about the framework before deciding whether to back the government.

Sir Jeffrey said Downing Street has indicated the vote on the Stormont brake will be read as indicative of parliament’s position on the wider deal.

Announcing the party’s intention ahead of Wednesday’s vote, he added: “Our party officers, the only decision-making mechanism in our party on these matters, met this morning and unanimously agreed that in the context of our ongoing concerns and the need to see further progress secured whilst continuing to seek clarification, change and re-working, that our Members of Parliament would vote against the draft statutory instrument on Wednesday.

“We will continue to work with the Government on all the outstanding issues relating to the Windsor Framework package to try to restore the delicate political balances within Northern Ireland and to seek to make further progress on all these matters.”