Former Attorney General Geoffrey Cox has said that he will vote against the government’s efforts to end the Brexit withdrawal agreement when it comes to the Commons.
He accused Boris Johnson of “subliminal” damage to Britain’s international reputation.
The Internal Markets Bill would go against the agreement signed by the UK and the European Union earlier this year.
Justice Secretary Robert Buckland said the bill was an “insurance policy”.
Mr Buckland told the BBC he hoped the powers being sought by ministers would never be needed, and that he would resign if UK international law “in a way I find unacceptable”.
Mr Cox supported Brexit and was the government’s top legal advisor at the time the withdrawal agreement was drafted.
Writing in the Times, Mr Cox said there was “no doubt” the “inconsistent” implication of the withdrawal agreement when the PM signed it.
“We, the British government and Parliament, have given our word. Our respect, our credibility, our self-esteem and our future effects in the world all of us rest by keeping that word,” Mr. Cox wrote.
He said there were legitimate ways for the government to deal with its concerns, such as using the procedure set out in the agreement to take “temporary and proportional measures” to protect the UK’s interests if approved by the Commons.
He said that ministers should not be provoked or frustrated, but should instead permanently and unilaterally use powers to rewrite portions of an agreement.
Britain has insisted that there should be no new investigation on goods going from Northern Ireland to Great Britain when it leaves the EU Single Market and Customs Union on 1 January.
The Northern Ireland Protocol is an important part of the withdrawal agreement signed by both sides last year, designed to prevent a difficult border returning to the island of Ireland.
The Internal Markets Bill would give ministers the power to reduce the amount of paperwork that Northern Irish firms would have to fill on goods bound for Great Britain, such as export and exit declarations, or to remove their need altogether.
It would also allow Britain to modify or re-interpret the “state aid” rules on subsidies for firms in Northern Ireland, in the event of the two parties not agreeing on future trade agreements.
This is controversial because it would change the terms of the Northern Ireland Protocol, a significant part of a legally binding withdrawal agreement by both parties before Britain’s exit from the European Union on 31 January.
Former Prime Ministers Tony Blair and Sir John Major have widely criticized the government’s intention, accusing Mr Johnson of “embarrassing” the UK.
The former leader held office during key times of the Northern Ireland peace process.
Both MPs rejected an “embarrassing” attempt to reject parts of the agreement, writing in the Sunday Times that the government’s action was “irresponsible, wrong in principle and dangerous in practice”.
The European Union has warned the UK that it could face legal action if it does not lose the controversial elements of the Internal Market Bill by the end of the month.
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