Last week, Northern Ireland Secretary Brandon Lewis told MPs that provisions in the Internal Market Bill would “break international law in a very specific and limited way”.

In an article for The Times, Mr Cox said that using the powers proposed in the bill to nullify “perfectly plain and foreseeable consequences” of the Withdrawal Agreement would “amount to nothing more or less than the unilateral abrogation of the treaty obligations to which we pledged our word less than 12 months ago, and which this parliament ratified in February”.

He added: “It is unconscionable that this country, justly famous for its regard for the rule of law around the world, should act in such a way.”

Although the EU is also “open to very severe criticism”, Mr Cox said it would be more appropriate to trigger the independent arbitration procedure set out in the Withdrawal Agreement.

“What ministers should not do, however provoked or frustrated they may feel, is to take or use powers permanently and unilaterally to rewrite portions of an agreement into which this country freely entered just a few months ago,” he said.

“Therefore, if the government does not urgently and effectively dispel the impression that it intends to do so, I shall have no choice but to withhold my support for this bill. I am a strong supporter of this government and of Brexit and I am deeply saddened to have to say this.

“We, the British government and parliament, have given our word. Our honour, our credibility, our self-respect and our future influence in the world all rest upon us keeping that word. Nothing less is worthy of Britain.”