Ministers believe they have largely muted Conservative opposition to the Northern Ireland protocol bill, even though one leading Conservative critic has said no MP should be voting for a breach of international law.

Leading opponents of Boris Johnson held off from publicly rejecting the legislation after it was published, despite the government’s fears beforehand that it would provoke a backlash.

Both Eurosceptics, who are examining the legislation line by line, and centrist one nation MPs largely kept their counsel. Sir Roger Gale, the North Thanet MP, was among the only Conservatives to express strong reservations, saying: “The legislation appears to be in breach of articles 26 and 27 of the Vienna convention on international treaties ratified by the UK in 1971. I don’t see how I or any member of parliament can vote for a breach of international law.”

Stephen Hammond, another Conservative MP and former remainer, also added his voice of criticism, saying: “Many colleagues are very concerned that this bill will breach international law and the commitments we have freely entered into … There is frustration about why now and how we are proceeding.”

However, the bulk of the 148 Conservative MPs who voted against Johnson’s leadership decided not to criticise the prime minister’s legislation, which has attracted a scathing response from Ireland and the rest of the EU.

One MP said the party was trying not to criticise the government in case it jeopardised the chances of Liz Truss, the foreign secretary, returning to the negotiating table, and that they were hoping the legislation would never have to come to a vote. “That would mean a difficult decision,” he said.

However, the EU sounded a negative note about starting negotiations again, repeating that the protocol cannot be changed, and has indicated it will restart legal infringement proceedings against the UK.

At the same time, the Democratic Unionist party is pushing for the legislation to be passed, before it will consider returning to power sharing. Sir Jeffrey Donaldson, the DUP leader, welcomed the Northern Ireland protocol bill on Tuesday, but said the party would revive the Stormont assembly only if the bill progressed at Westminster.

“Parliament can either choose to go forward with the [Good Friday] agreement and the political institutions and stability in Northern Ireland, or the protocol, but it can’t have both,” he told BBC radio’s Good Morning Ulster.

The government had justified overturning swaths of the protocol, which formed part of the divorce deal with the EU, on the grounds that it was necessary to restore power sharing in Northern Ireland.

“People deserve to have a government,” Truss told the BBC. “We have published this bill, which does deliver for all the communities of Northern Ireland.” Power sharing should resume “as soon as possible”, she said.

However, Donaldson made clear that his party would budge only after the legislation made progress with MPs. “There is a stark choice here for parliament. The Northern Ireland protocol and Good Friday agreement cannot exist together. One seriously harms the other. The protocol undermines the cross-community consensus on which the political institutions operate.”

Peter Robinson, a former DUP leader who retains influence, underlined the hardline mood with an op-ed in the Belfast News Letter. “The DUP should remain outside the executive until the bill is delivered,” he wrote. “It is only the DUP’s refusal to enter the executive until this issue is resolved that has provided the necessary movement and momentum, the party must act with caution and certainty.”

Legal opinion also coalesced against the government’s claim to justify the breaking of international law under the “doctrine of necessity”.

Dr Ronan Cormacain, senior research fellow at the Bingham Centre for the Rule of Law, said his initial view was that he “struggles to see any possible way that the arguments set out by the government clear the high hurdles in article 25 legitimising a breach of an international treaty”.

“It isn’t necessary,” he said. “It isn’t the only way that interests can be safeguarded. There is no grave and imminent peril. It seriously impairs the interests of the other parties to the protocol.

“It unilaterally breaches an agreement the UK made with the EU, ostensibly to protect the people of Northern Ireland, whilst completely ignoring what the majority of the people in Northern Ireland want.”