Sammy Wilson has said the implementation of the revised post-Brexit trade deal is the beginning of process that will see the Northern Ireland's position within the UK "weakened".
The DUP MP was speaking as key aspects of February's Windsor Framework deal become operational this weekend.
"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," the East Antrim representative said.
On Thursday, British Prime Minister Rishi Sunak said the framework removed the major stumbling block to the restoration of the Stormont institutions. However, Mr Wilson said DUP would not go back into government to implement the revised agreement.
"If we were inside Stormont, the law now requires us, and judgments have been made in the courts, that we implement it," he told the BBC.
The DUP is seeking assurances in legislation from the British government which safeguards the north's place in the UK internal market.
Despite months of negotiations, no breakthrough is in sight.
Windsor Framework implementation confirms Irish Sea border, DUP MP says
Sinn Féin's Declan Kearney said the framework's implementation created an opportunity to "move forward and make progress".
On Thursday, Sinn Féin deputy leader Michelle O'Neill said patience had run out with the DUP over its Stormont boycott and she called on the two governments to work on a plan to restore devolution.
Mr Kearney said he welcomed a commitment by both the British government and the EU earlier this week that they would fully implement the Windsor Framework in a faithful way.
“It’s now time to move forward – we now have unprecedented economic opportunities to strengthen our economy and create more and better jobs," he said.
"We need to see the assembly and executive working now to address the problems in our health service and deliver for business, for workers and families."
SDLP leader Colum Eastwod said the EU-UK deal provided a "clear path back to devolved government", alongside economic opportunities.
"The SDLP has substantial concerns about the operation of the 'stormont brake' but the alternative – no government, hospital waiting lists growing, the ecological crisis at Lough Neagh deepening, schools starved of funding and struggling families crushed by the cost of living – is a total abdication of responsibility by political leaders," he said.
Alliance deputy leader Stephen Farry described the framework as an "upgrade from the original protocol".
"While we acknowledge that it is not perfect, it will be more light touch than other alternatives," the North Down MP said.
"We do not want to see any new borders or friction within these islands, but this is an outworking of Brexit and the shape of that Brexit chosen by others."
