British Prime Minister Boris Johnson says his plan to unilaterally rewrite Britain's divorce deal with the European Union is an insurance policy against the bloc's unreasonable behavior, comments that came as his former attorney general joined a growing number of once-loyal lawmakers condemning the contentious move.

Johnson said Monday that a planned law designed to override portions of the Brexit withdrawal agreement was needed because the EU might "go to extreme and unreasonable lengths" in its treatment of former member Britain.

"I have absolutely no desire to use these measures," Johnson told lawmakers as he introduced the Internal Market Bill in the House of Commons. "They are an insurance policy."

The U.K. formally left the bloc on Jan. 31, but existing trade rules remain in effect until the end of this year under a transition designed to provide time to negotiate a long-term trade agreement.

Johnson's Conservative government has acknowledged that the bill breaches the legally binding withdrawal treaty that Britain and the EU have both ratified. The legislation threatens to sink the already-foundering negotiations between Britain and the EU on a post-Brexit trade deal.

Ed Miliband, business spokesman for the opposition Labour Party, accused Johnson of "trashing the reputation of this country and trashing the reputation of his office."

With an 80-seat majority in the House of Commons, Johnson is expected to have enough votes to push his legislation through Parliament but there is wide unease within the Conservative Party about the law-breaking move.

Geoffrey Cox, who was the government's top legal officer when Johnson negotiated the Brexit withdrawal agreement less than a year ago, said reneging on the deal would be an "unconscionable" breach of international law.

Cox, previously a strong supporter of Johnson on Brexit, said he wouldn't support the proposal at its first House of Commons vote on Monday.

"I simply cannot approve or endorse a situation in which we go back on our word, given solemnly," Cox said on Times Radio. "The breaking of the law ultimately leads to very long-term and permanent damage to this country's reputation."

Sajid Javid, a former Treasury chief in Johnson's government, also said he would not vote for the bill, because "I cannot support the U.K. pre-emptively reneging" on the withdrawal agreement.

The U.K. formally left the bloc on Jan. 31, but existing trade rules remain in effect until the end of this year under a transition arrangement designed to provide time to negotiate a long-term trade agreement.

As part of the Brexit divorce deal, Britain and the EU agreed to keep Northern Ireland -- the only part of the U.K. to share a border with the bloc -- bound to some EU rules on trade, to avoid the need for border checks on goods moving between Northern Ireland and the Republic of Ireland. Both sides accepted the compromise to protect the open border, which helps underpin the peace process in Northern Ireland.

E.U. DEMANDS U.K. REWRITE BREXIT BILL, THREATENS LEGAL ACTION

The Internal Market Bill, which the government hopes to pass into law within weeks, would give the British government the power to override the EU's agreed role in oversight of trade between Northern Ireland and the rest of the U.K.

Johnson claims the EU has threatened to use "an extreme interpretation" of the withdrawal agreement to "blockade" food shipments from the rest of the U.K. to Northern Ireland unless Britain agrees to accept EU regulations.

He said the bloc was "threatening to carve tariff borders across our own country and to divide our own land."

The EU denies threatening a blockade and says it merely wants Britain to live up to the terms of the agreement. EU leaders are outraged at the prime minister's proposal and have threatened the U.K. with legal action if it does not drop the proposal by the end of the month.

Two former Conservative U.K. prime ministers, John Major and Theresa May, have condemned the legislation. On Monday a third, David Cameron, said he had "misgivings."

Conservative lawmaker Rehman Chishti quit Monday as the prime minister's special envoy on freedom of religion in protest over the bill. He tweeted that as a former lawyer, "values of respecting rule of law & honouring one's word are dear to me."

What mystifies some observers is that Johnson is repudiating a treaty that he himself negotiated and hailed as an "oven-ready" deal that would "get Brexit done." That declaration of victory was key to Johnson's successful December 2019 election campaign.

"There was a political imperative on the government to get an agreement and then to go to the electorate with the claim that they had, to coin a phrase, got Brexit done," said Tim Bale, professor of politics at Queen Mary University of London.

"I think it possibly was the case in some senses that it was `make the agreement in haste and then repent at leisure.' And what we're seeing now is the repentance."

Johnson's move has chipped away at the dwindling trust between Britain and the EU as they try to negotiate a new trading relationship.

Despite the chill in relations between London and Brussels and the threat of legal action, trade talks between the two sides are to continue this week. Both sides say any deal must be agreed by next month so there is time for it to be ratified by Dec. 31.

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If there is no deal, tariffs and other impediments to trade will be imposed by both sides at the start of 2021.

That would mean huge economic disruption for the U.K., which does half its trade with the bloc. A no-deal exit on Jan. 1 would also hit some EU nations, including Ireland, France, Belgium and the Netherlands, especially hard.