A cabinet minister has admitted Boris Johnson’s plan to change his own Brexit deal breaks international law.

Northern Ireland secretary Brandon Lewis said the proposals to change the operation of his own withdrawal agreement (WA), which smoothed the UK’s departure from the EU in January, breaks the law in a “very specific and limited way”.

The plan threatens to collapse Brexit negotiations on a trade deal and have left the prime minister’s predecessor Theresa May questioning whether the UK can be “trusted” internationally. 

Johnson is trying to unpick key sections of the WA on trade in Northern Ireland, despite negotiating the provisions himself to make the deal initially negotiated by his predecessor May more palatable to the Conservative Party.

The prime minister’s plan would give UK ministers key powers to decide when EU rules apply to Northern Ireland, in apparent defiance of his own deal.

During an urgent question on the plan, veteran Tory Sir Bob Neill, who chairs the Commons justice committee, told Lewis that “adherence to the rule of law is not negotiable”.

He went on: “Against that background, will he assure us that nothing that is proposed in this legislation does or potentially might breach international legal obligations or international legal arrangements that we have entered into?”

Lewis replied: “Yes this does break international law in a very specific and limited way.”

The exchange came after the head of the government’s legal department Jonathan Jones quit, reportedly over Johnson’s plans for the WA.

Lewis refused to comment on the reasons for Jones’ resignation.

In a message to staff, Jones reportedly did not address directly address the issue but thanks the legal department for wrestling with the “unique legal and constitutional complexities of leaving the EU”.

IN DEPTH Is Boris Johnson Really About To Blow Up The Brexit Negotiations?

In the Commons, Lewis went on: “We are taking the power to disapply the EU law concept of direct effect, required by Article 4 in certain, very tightly defined circumstances.

“There are precedents for the UK and indeed other countries needing to consider their international obligations as circumstances change.”

Lewis said the Finance Act 2013 “contains the example of treaty override, it contains provisions that expressly disapply international tax treaties to the extent that these conflict with the general anti-abuse rule.”

The proposals drew sharp criticism from May, who has long found Johnson a thorn in her side.  

Johnson quit May’s cabinet over Brexit and eventually usurped her as Tory leader and PM.

In the Commons, May said: “The United Kingdom government signed the withdrawal agreement with the Northern Ireland protocol.

“This parliament voted that withdrawal agreement into UK legislation.

“The government is now changing the operation of that agreement.

“Given that, how can the government reassure future international partners that  the UK could be trusted to abide by the legal obligations of the agreements it signs?”

Northern Ireland was supposed to follow EU rules on certain aspects of trade but UK rules on others, to ensure it keeps an invisible border with the Republic.

But the government now wants to give UK ministers the power to decide what goods are “at risk” of being sold into the EU via Northern Ireland, and must therefore comply with Brussels tariffs.

Ministers would also have the power to decide when state aid – government support for businesses – should be notified to Brussels because it affects Northern Ireland, and by extension the EU. 

It has been seen as potentially reneging on the WA’s Northern Ireland protocol.

Lewis insisted the provisions were simply a “safety net” to ensure there is no legal confusion if the UK and EU, through the so-called “joint committee” that monitors the implementation of the WA, cannot agree on Northern Ireland. 

Responding to May, he said: “We have worked with the EU in the spirit of good faith and I know we continue to do that, both sides working in a spirit of good faith, to ensure we do implement arrangements which uphold the fundamental principles that lie behind the protocol.

“Our first priority continues to be to secure agreement on the protocol through the joint committee and the wider free trade agreement.

“But the withdrawal agreement protocol is not like any other treaty. It was written on the assumption that subsequent agreements could be reached between us and the EU on the detail.

“That is the entire purpose of the specialised joint committee.

“We continue to believe that that is possible but as a responsible government we cannot allow businesses to not have certainty for January.”

Shadow Northern Ireland secretary Louise Haigh, who asked the urgent question, said Lewis’s admission was “astonishing”

Liberal Democrats leader Sir Ed Davey tweeted: “This is a really sad and shocking state of affairs for our country. Breaking international law will do untold damage to our reputation abroad, it will make us poorer and make it harder to solve global crises like the climate emergency.”