The Government has been warned by the DUP leader Sir Jeffrey Donaldson that the Irish Sea border needs to be removed “for the future of the union”.

The leaders of Northern Ireland’s three main unionist parties shared a platform to strongly criticise the Northern Ireland Protocol at a fringe event at the Conservative Party conference on Monday.

Sir Jeffrey also met with the Prime Minister where he reiterated his call for action by the Government within weeks.

The protocol was agreed by the UK and EU as a way to maintain a free-flowing land border on the island of Ireland after Brexit.

Unionists in Northern Ireland are opposed to its terms, which see additional checks on goods arriving in the region from the rest of the UK, regarding it as a border in the Irish Sea.

Last month, Sir Jeffrey threatened to collapse the powersharing institutions at Stormont if his demands on the protocol were not met.

He told the Manchester fringe event: “We simply cannot allow this situation to continue. And we need to see action taken by the Government within weeks.

“What we need now is to move beyond the words of the command paper published in July which were good and provided a road map for a way forward. We need action.

“We need the Government to sit up, and to take action to remove this Irish Sea border, remove the barriers to trade within the United Kingdom, and fundamentally to restore Northern Ireland’s place within the UK internal market. At the heart of this is the future of the union itself.”

On his threat to collapse Stormont, Sir Jeffrey said: “I cannot sustain a situation where we are required to use the political institutions in Northern Ireland to implement the protocol that is designed to undermine our place within the union.”

Earlier, Brexit minister Lord Frost delivered a speech to the conference where he said if the UK and the EU cannot strike an agreement, Britain will consider what is seen to be the nuclear option of triggering Article 16.

Sir Jeffrey responded: “Triggering Article 16 has its use in the short term, but what we require in order to restore Northern Ireland’s place fully within the United Kingdom is UK legislation.”

The DUP leader was joined at the event by Ulster Unionist leader Doug Beattie, TUV leader Jim Allister and former UUP leader Lord Trimble.

Mr Beattie told the fringe event that he did not agree with the threat to bring down the Stormont institutions.

He said: “I do not think collapsing the institutions is good for Northern Ireland. I do not think it is good for the people.

“I cannot look at the people in the face, those 300, 000 on hospital waiting lists and say we are going to collapse an institution that is supposed to be looking after you.”

Mr Allister told the meeting that the protocol had separated Northern Ireland from the rest of the UK.

He said: “In Manchester today – under the principles and purposes of the protocol- I am in a ‘third country’ as far as protocol ruled Northern Ireland is concerned.

“That is an affront to our supposed status as an integral part of the United Kingdom and an intolerable affront to our Britishness. Is it an affront to the Conservative and Unionist Party?”

He added: “Brexit was supposed to be about ‘taking back control’; in our case it was about handing away control. We did not get Brexit, we got the Brexit-denying Protocol.”