Former attorney general Geoffrey Cox has said he will vote against the government’s attempts to override the Brexit Withdrawal Agreement when it comes before the Commons.
He accused Boris Johnson of doing “unconscionable” damage to Britain’s international reputation.
The Internal Market Bill would go against the agreement signed by the UK and European Union earlier this year.
Justice Secretary Robert Buckland said the bill was an “insurance policy”.
Mr Buckland told the BBC he hoped powers being sought by ministers would never be needed, and that he would resign if the UK ended up breaking international law “in a way I find unacceptable”.
Mr Cox backed Brexit and was the government’s top legal adviser when the Withdrawal Agreement was drawn up.
Writing in The Times, Mr Cox said there was “no doubt” the “unpalatable” implications of the Withdrawal Agreement were known when the PM signed it.
“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,” Mr Cox wrote.
He said that there were lawful ways for the government to deal with its concerns, such as using a procedure set out in the agreement to take “temporary and proportional measures” to protect the UK’s interests if approved by the Commons.
“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 added.
The UK has insisted there must be no new checks on goods moving from Northern Ireland to Great Britain when it leaves the EU’s single market and customs union on 1 January.
The Northern Ireland Protocol, a key part of the Withdrawal Agreement signed by both sides last year, is designed to prevent a hard border returning to the island of Ireland.