Brexit: David Cameron joins former PMs warning against Boris Johnson's bill

David Cameron has become the third former Conservative prime minister to express disquiet about Boris Johnson’s proposal to breach international law by unilaterally redrafting part of the Brexit deal with the EU, saying he had “misgivings” about the idea.

In comments on Monday, before MPs began debating the internal market bill that sets out the plans, Cameron said: “Passing an act of parliament and then going on to break an international treaty obligation is the very, very last thing you should contemplate.

“It should be an absolute final resort. So, I do have misgivings about what’s being proposed.”

Theresa May and John Major have warned in stronger terms about the plans, saying they could damage the UK’s international reputation, as has another former Conservative leader, Michael Howard.

Cameron said he also had to consider that the UK was “in a vital negotiation with the EU to get a deal and I think we have to keep that context, that big prize in mind”.

He added: “And that’s why I have perhaps held back from saying more up to now.”

The comments come amid a potential Tory backbench rebellion about the plan, with the former attorney general Geoffrey Cox saying on Monday that breaking international law risked causing “very long-term and permanent damage to this country’s reputation”.

Related: Brexit bill criticised as 'eye-watering' breach of international law

Ahead of the second reading of the bill, the first opportunity MPs will have to debate it, Cox said he understood government arguments that the EU was acting in bad faith over the Brexit withdrawal agreement.

“But what you can’t do, and what I think is wrong, is to abandon an agreement, to rewrite unilaterally parts of an agreement, which you only signed nine months ago, and to which we have given our solemn word,” he told Times Radio.

He added: “The breaking of the law, ultimately, leads to very long-term and permanent damage to this country’s reputation. And it’s also a question of honour, to me. We signed up, we knew what we were signing. We simply can’t seek to nullify those ordinary consequences of doing that.”

Cox did, however, indicate that while he could not support the bill as it stood, this could change: “If the government were to say that these powers will only be used in these specific circumstances, where it would be lawful to act in this way, then that might well be a different position. But I haven’t had those assurances yet.”

Hours after Cox’s comment, another Brexit-backing Tory MP and former barrister, Rehman Chishti, a key backer of Johnson in the Tory leadership contest, also said he could not support the bill.

Chishti resigned as the prime minister’s envoy on religious freedom in order to withdraw his support for the legislation, underlining how uncomfortable many current and former lawyers on the Conservative benches are with the proposed bill.

“I will not be able to support this bill as a matter of principle. I have real concerns with the UK breaking its legal commitments under the withdrawal agreement,” he said.

“During my 10 years in parliament and before that as a barrister, I have always acted in a manner which respects the rule of law. I feel strongly about keeping the commitments we make, if we give our word, then we must honour it. Voting for this bill as it currently stands would be contrary to the values I hold dearest.”

Defending the government’s stance, the policing minister, Kit Malthouse, said that even with his job he had no qualms about supporting a measure that potentially broke international law.

“I’m policing minister, so I’m responsible for the criminal law, and this is obviously a civil matter and an international law matter,” he told BBC1’s Breakfast programme.

Malthouse argued that the action was needed because the EU had threatened to potentially threaten food exports to Northern Ireland from Great Britain, which Brussels has rejected.

He said: “The lawyers will bat it backwards and forwards, I have absolutely no doubt about that. But from my point of view, as a non-lawyer, I’m looking at the practical effect.”

Malthouse rejected the idea that the government planning to break the law could prompt people to ignore UK laws, for example new rules on coronavirus: “We think it is a good example.”

Labour’s shadow business secretary, Ed Miliband, disagreed, saying: “How can we on the one hand be saying you’ve got to obey the law, which we all say rightly as legislators, and then the government comes along and says, well it’s OK for us to break the law because it’s specific and limited. We can’t be having that.”

The Labour leader, Sir Keir Starmer, echoed this on his monthly call-in appearance on LBC radio. “Here we are, on the world stage for the first time in many years on our own, and what’s the first thing we do? We break a treaty,” he told one listener.

Starmer said many people would be baffled at Johnson’s row with the EU: “I think the vast majority of the population would say, ‘What on earth is going on? You’re reopening things that we thought were closed. You said you’d get a deal, get on and get a deal.’ ”