Former Attorney General Jeffrey Cox has said he will vote against the government’s efforts to violate the Brexit withdrawal agreement when it comes before the Commons.
He accused Boris Johnson of causing “inconsistent” damage to Britain’s international reputation.
The domestic market bill will go against the agreement signed by the UK and the EU earlier this year.
Judicial Secretary Robert Buckland introduced the bill Is an “insurance policy”.
Mr Fuckland told the BBC “I do not consider it necessary to comment on such fabrications” and that he would resign if the UK ended up violating international law.
Mr Cox supported 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 that the “irrelevant” implications of the withdrawal agreement were known when the Prime Minister signed it.
“We, the British Government and Parliament have given our word. Our respect, credibility, our self-esteem and our future influence in the world are all in keeping with that word,” Mr Cox wrote.
He said there were legal ways to address the government’s concerns, such as using the procedure set out in the agreement to take “temporary and proportional measures” to protect the interests of the UK if the Commons approves it.
“What ministers should not do, no matter how provoked or frustrated, is to take over the powers permanently and unilaterally or to rewrite parts of an agreement that this country entered into freely a few months ago,” he added.
The UK has insisted that there should be no new tests on goods moving from Northern Ireland to Great Britain as it exits the EU single market and customs on January 1.
The Northern Ireland Protocol, a key component of the withdrawal agreement signed by both sides last year, is designed to prevent a difficult border return to the island of Ireland.
The Internal Market Bill would give ministers the power to reduce or completely eliminate the need for Northern Irish companies to fill out items such as export and exit notices for Great Britain.
In the event of disagreement between the two parties on a futures trade agreement, the UK will be allowed to modify or redefine the “Government Assistance” rules on subsidies to companies in Northern Ireland.
This is controversial because it would change the terms of the Northern Ireland Code of Conduct, a key part of the legal withdrawal agreement agreed by both sides before the UK’s exit from the EU on 31 January.
The government’s motives have drawn widespread criticism, with former prime ministers Tony Blair and Sir John Major accusing Mr Johnson of “embarrassing” England.
Former leaders were in office during key periods of the Northern Ireland peace process.
Both urged MPs to reject “shameful” attempts to breach certain parts of the agreement. Writes in the Sunday Times The actions of the government are “irresponsible, wrong in principle and dangerous in practice”.
The European Union (EU) has warned the United Kingdom that it will face legal action if the controversial elements of the domestic market bill are pushed back by the end of this month.