Brexit: David Cameron joins all living former PMs in condemning Boris Johnson’s plan to break law

Gordon Brown, Theresa May, Tony Blair and John Major have also spoken out

David Cameron has  criticised Boris Johnson’s plan to break international law, warning that ignoring a treaty is “the very, very last thing you should contemplate”.

Mr Cameron joins John Major, Theresa May, Gordon Brown, and Tony Blair in  condemnation, meaning all former living prime ministers have now urged the PM to change course.

“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 is being proposed,” he said.

MPs on Monday are debating the Internal Market Bill, which includes provisions that violate the Brexit withdrawal agreement signed by Boris Johnson last year.

Critics say the move will undermine the UK’s standing in the world and make it harder to criticise other countries that shirk their obligations.

At the weekend Mr Blair and Sir John said the policy was "irresponsible, wrong in principle and dangerous in practice". Theresa May said the move could damage “trust” in the UK while Gordon Brown branded it an act of “self-harm”.

Mr Cameron said the priority at the moment for the government should be getting a deal, adding: "I would just make this point: so far what's happened is the government's proposed a law that it might pass or it might not pass or might use or might not use depending on whether circumstances do or do not appear. 

“The bigger picture here is we are in a vital negotiation with the European Union in order to get a deal and I think we have to keep that context, that big prize in mind, and that's why I've perhaps held back from saying more up till now.”

Justice Secretary Robert Buckland has said the powers being sought by the government to amend aspects of the Northern Ireland Protocol are an "insurance policy".

The government says they will only be used if their attempts to settle differences in other ways fail. However, the withdrawal agreement already includes an arbitration procedure to settle disputes, and also clearly states that the UK must apply the EU customs code in Northern Ireland – which the UK now wants to get out of.

The EU customs code which the UK signed Northern Ireland up to requires so-called “export summary declarations” for goods leaving its jurisdiction, but Boris Johnson has now said he does not want Northern Ireland businesses to have to fill in paperwork when shipping goods to Great Britain.