Rishi Sunak’s hopes of ending years of Brexit infighting with a revised deal for Northern Ireland have suffered a double blow as Boris Johnson came out against the plan while pressure mounted within the Democratic Unionist party (DUP) to reject it.

In his first public comments since the Windsor framework was unveiled on Monday, Johnson used a speech to a conference in London to say he would find it “very difficult” to back the plan, arguing it would stifle the UK economically.

While Johnson’s opposition was expected, and the former prime minister did not explicitly say he would vote against the new protocol when it reaches the Commons, his careful unpicking of a policy he said would yoke the UK to European-style economic orthodoxy will strike a chord with fellow Tory Brexiters.

“We must be clear about what is really going on here. This is not about the UK taking back control,” he said of Sunak’s plan. “This is the EU graciously unbending to allow us to do what we want in our own country, not by our laws, but by theirs.”

This would, Johnson argued, “act as a drag anchor on divergence – and there’s no point in Brexit unless you do things differently”.

He also called on Sunak not to drop a bill, currently going through parliament, that would allow the UK to unilaterally change elements of the Northern Ireland protocol, arguing this was the best way to win concessions from the EU.

On the Windsor plan, Johnson said: “I’m going to find it very difficult to vote for something like this myself because I believed that we should have done something different, no matter how much plaster came off the ceiling in Brussels.

“I hope that it will work. And I also hope that if it doesn’t work, we will have the guts to deploy that bill again. Because I’ve no doubt at all, that that was what brought the EU to negotiate seriously.

Sunak’s proposal, which still includes some EU oversight of trade in the UK but seeks to placate unionists with the “Stormont brake”, which allows Northern Ireland’s devolved assembly to block new EU regulations in some circumstances, has been generally well received by Tory MPs, including many Brexiters.

However, the European Research Group, which represents the most diehard Brexit opinion, has yet to give a formal view while it fully studies the legal text of the deal. The same is true of the DUP, which has set out seven tests the plan must comply with.

The DUP leader, Sir Jeffrey Donaldson, has said the decision on whether to support it will be “collective”, with most expecting him to take two weeks to consider independent legal opinion.

But some observers worry that the vacuum left during the wait for an opinion from Donaldson is allowing opponents of the deal within the party to build up momentum, notably Sammy Wilson, its Westminster chief whip, and senior MP Ian Paisley.

“I don’t believe it meets our tests. And there’s probably six or seven reasons why – for example, EU law will continue to apply in Northern Ireland,” Paisley told BBC One Northern Ireland’s Nolan Live programme.

One issue was that the Stormont brake “only applies to future law, not to existing EU law”, Paisley said, adding: “This is not, of course, a legal agreement. This is a political statement.”

Sunak was spending Thursday at the same hotel in Windsor where he finalised his new Brexit plan with Ursula von der Leyen, the European Commission president, for a long-planned awayday with Conservative MPs, one intended to push key messages and build unity.

The impact of Johnson’s thumbs down remains to be seen, but in an uncharacteristically reflective section of his speech he appeared to accept that his arguments over embracing divergence from EU economic orthodoxies seemed unlikely to happen.

“People wanted change in their lives. They wanted to see things done differently,” he said.

“I’ve got to put my hands up for this as much as anybody. We haven’t done enough yet to convince them that it can deliver the change they want to see.

“What I wish we had done is put a big ‘Invest here’ sign over Britain as soon as we were out of Covid.”