Rishi Sunak must stand up to the “Brexit purity cult” of Eurosceptics on the Tory benches to find a solution to the Northern Ireland Protocol issues, Sir Keir Starmer will say.

The Labour leader will use a speech in Belfast to urge the Prime Minister to take on the European Research Group (ERG) in order to resolve the issues over Northern Ireland’s post-Brexit trading arrangements.

Sir Keir will say he will offer “political cover” at Westminster if Mr Sunak can deliver an agreement with the European Union which is in the national interest.

“The time for action on the protocol is now. The time to stand up to the ERG is now,” Sir Keir will say on Friday.

“The time to put Northern Ireland above a Brexit purity cult, which can never be satisfied – is now.”

There was a “small window of opportunity” to resolve the issue before the 25th anniversary of the Good Friday Agreement in April.

“We’ve got to use the anniversary to fix minds, get the country and its political process moving forward again – deliver for the people of Northern Ireland,” he will say.

Any deal seen as conceding too much to Brussels could trigger a revolt on the Tory benches, but Sir Keir will offer Labour support to Mr Sunak.

“Whatever political cover you need, whatever mechanism in Westminster you require, if it delivers for our national interest and the people of Northern Ireland, we will support you,” he will say in the speech at Queen’s University.

The Labour leader will call on the Prime Minister to recognise the mistakes made by some Tory ministers who had viewed the Irish government as “adversaries” on Brexit.

“That has damaged the political process here in Northern Ireland – no question. And it’s certainly not the spirit of 1998.”

The Northern Ireland Protocol was agreed by then-prime minister Boris Johnson to avoid a hard border with Ireland but the DUP is refusing to engage with the powersharing institutions until it is dramatically altered or removed.

Unionists oppose the trade barriers it has created between Northern Ireland and the rest of the UK.

Labour has suggested a new veterinary agreement for trade in agricultural products between the UK and EU, which it claims would eliminate many of the checks, while a trusted trader scheme could also avoid some bureaucracy.

Sir Keir will also use the speech to hail the Good Friday Agreement as ”the greatest achievement of the Labour Party in my lifetime, without question”.

“But of course, the real achievements – the real pride – belongs to the people and communities here in Northern Ireland,” he will say.