The implementation of a key element of the Windsor Framework this weekend will confirm a border in the Irish Sea, a DUP MP has said.
Sammy Wilson also said his party cannot currently return to the powersharing institutions at Stormont as it would be legally required to implement the framework.
The DUP has been blocking powersharing for more than a year in protest at the internal UK trade barriers created by Brexit’s Northern Ireland Protocol.
The party says the deal struck by the EU and the UK to reform the protocol – the Windsor Framework – does not sufficiently address its concerns and has made clear it will not accept a return to devolution until the Government provides further assurances, by way of legislation, over Northern Ireland’s place in the UK internal market.
Talks between the DUP and the British government have been ongoing over the summer.
One of the main parts of the framework – the green/red lane system for the movement of goods – will become operational at Northern Ireland ports on Sunday.
Goods coming into the region which are travelling to the Republic or elsewhere in the EU will use the red lane, which includes customs declarations and some checks.
Goods to be sold in the North will use the green lane with minimal paperwork and no checks.
Companies that use the green lane will be signed up to a trusted trader scheme.
“On Sunday it will be confirmed that Northern Ireland has got a border in the Irish Sea,” Mr Wilson told the BBC Good Morning Ulster programme.
“Despite what the prime minister said about the Windsor Framework, despite what the Secretary of State has said, we are going to have goods of all nature checked to some extent coming into Northern Ireland.
“We are going over the next year to see physical border checkpoints built in a six-acre site in Larne, £40 million will be spent on a border checkpoint to mark Northern Ireland as being part of the EU rather than being fully part of the United Kingdom.
“That has implications for the goods which come into Northern Ireland, the cost of those goods, the cost of transporting them, the variety of goods which will be available, so consumers will feel that impact.
“Over a period of time we will see the laws in Northern Ireland diverging from the rest of the United Kingdom and our position within the United Kingdom weakened.”
Prime Minister Rishi Sunak said on Thursday that the agreement of the framework removed the major stumbling block to the return of the Stormont Assembly.
But Mr Wilson said his party would not go back into government to implement the agreement.
He said: “From Sunday there will be impediments to trade coming into Northern Ireland.
“There will be different laws applying in Northern Ireland without any democratic control, because EU law will apply to Northern Ireland, so when he (Rishi Sunak) says that the union is safe, that he has safeguarded the union, he has taken away any sense of a border in the Irish Sea, that is not true.
“If we were inside Stormont, the law now requires us, and judgments have been made in the courts, that we implement it.
“There should have been a £40 million six-acre border checkpoint built in the Port of Larne by now, the DUP minister was able to stop that because he refused to give permission for it.
“If he were back in Stormont tomorrow he would by law be required to facilitate that, to introduce all the measures to do it, the procurement, appointing the builders.
“That is the reason why we will not and cannot as unionists be in Stormont where we are required to implement an agreement we believe is detrimental to the union and to the people of Northern Ireland.”
On Thursday, Sinn Féin vice president Michelle O’Neill said patience had run out with the DUP blockade of the Assembly.
She called on the UK and Irish governments to work together on a plan to restore the institutions.