Opposition parties have demanded ministers release their full legal advice over a bill to unilaterally amend the Northern Ireland protocol that is expected imminently in the Commons, saying refusing to do so risked accusations of a cover-up.

The bill, which sets the UK on a potential collision course with the EU and which critics see as Boris Johnson’s latest attempt to mollify rebellious backbenchers and reassert his authority, is scheduled to be published on Monday afternoon.

Downing Street officials said the government had received full advice on whether a one-sided attempt to change the deal risked breaching international law, but that it only planned to publish a summary.

The Northern Ireland secretary, Brandon Lewis, said the bill was intended to protect the integrity of the Good Friday peace agreement and that when people saw the legislation they would understand it did not contravene international law.

Asked whether the full advice would be published, he told Sky News’s Sophy Ridge on Sunday show only that the government would be “outlining our legal position”.

Quizzed three times about whether Sir James Eadie, the senior barrister whose role as first Treasury counsel involves giving ministers independent legal advice, had been asked about the bill, Lewis declined to say.

“I’m not going to get into the internals of government advice,” he said. Pressed further, he said: “The government lawyers are very clear that we are working within the law. The attorney general will be setting out the government’s position on that tomorrow.”

The shadow Northern Ireland secretary, Peter Kyle, said the bill had “potential for malicious and rogue governments to interpret it as a green light for unilateral action against international treaties to which they are bound”.

He added: “Given this, it is incumbent on ministers to release the maximum possible legal advice from the start, so the legal basis upon which they make their case to parliament can be judged.”

The Lib Dems’ spokesperson on Northern Ireland, Alistair Carmichael, said: “The refusal to publish this legal advice looks like yet another attempt to cover up Boris Johnson’s repeated lies and law-breaking.

“The government must come clean and publish what legal advice was received and who from in full.

“The public deserves full transparency over the legality of plans to rip up the Northern Ireland protocol and risk a trade war with our closest neighbours. If Conservative ministers have nothing to fear, they have nothing to hide.”

The bill will unilaterally override elements of the post-Brexit protocol with the EU to try to make trade easier between Northern Ireland and the rest of the UK, something Brussels has said could spark retaliation.

The hardline Eurosceptic right of the Tory party have put ministers under pressure to take tough action, with MPs having held meetings with the foreign secretary, Liz Truss.

Johnson is seen as more amenable to their message as he tries to court backbenchers after last Monday’s confidence vote in which 41% of his MPs tried to oust him.

Lewis said: “What we’re looking to do is to fix the problems we’ve seen with the protocol. It’s about how the protocol has been implemented, the lack of flexibility we’ve seen from the EU over the last year and a half.”

But the Sinn Féin leader, Mary Lou McDonald, said a significant majority of Northern Ireland assembly members elected in May backed the protocol, and that it was clear ministers in Westminster intended to break the law.

“The protocol is working,” she told Ridge’s show. “The protocol is the mechanism that gives the north, uniquely, unfettered access to the European market. That is why we see in the north of Ireland, in contrast with Britain, with the exception of the City of London, the economy is strong.

“What the Tory government is proposing to do in breaching international law is to create huge, huge damage to the northern economy, to the Irish economy.”

The shadow chancellor, Rachel Reeves, told Ridge she was concerned the plan would contravene international law.

“We haven’t seen the legislation yet, but it does look like the government plans to break international law,” she said. “This government seems to be developing a record for law-breaking, and it’s not one that the Labour party can support.

“We helped bring in the Good Friday agreement. We are deeply, passionately committed to it.”