The Tory MP said it was “unconscionable” that the UK should seek to break international law by rewriting the withdrawal agreement with the European Union.
Writing in The Times, Mr Cox, who backed the Leave campaign, warned that he would not back the UK Internal Market Bill unless ministers dispel the impression they plan to “permanently and unilaterally” rewrite an international agreement.
The QC, who resigned in February at the prime minister’s request, said tariffs and customs procedures on certain goods entering Northern Ireland from Britain were part of the deal.
“There can be no doubt that these were the known, unpalatable but inescapable, implications of the agreement,” he added.
He said if the powers in the bill were used to “nullify those perfectly plain and foreseeable consequences” then it would amount to the “unilateral abrogation of the treaty obligations” signed in October.
“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,” he said.
Mr Cox urged ministers to use the “clear and lawful” options under the agreement to remedy their concerns that food imports may be blocked from Britain to Northern Ireland.
Or, “in extremis” he said, they could take “temporary and proportionate measures” during an independent arbitration process.
“What ministers should not do, however provoked or frustrated they may feel about an impasse in negotiations, is to take or use powers permanently and unilaterally to rewrite portions of an international agreement into which this country freely entered just a few months ago,” he said.
His intervention came ahead of MPs debating the legislation on Monday, when the bill returns to the Commons amid growing criticism that breaching international law would jeopardise the UK’s standing in the world.
The current attorney general, Suella Braverman, and justice secretary Robert Buckland, have both been heavily criticised for their roles during the controversy.
Mr Buckland told the Andrew Marr Show that he would only resign “if I see the rule of law being broken in a way I find unacceptable” and insisted ministers were committed to getting a trade deal with the EU.
It comes after the prime minister warned that Brussels could “carve up our country” without his new bill and impose a food blockade in the Irish Sea.
Outrage at the bill has come from across the political spectrum, including from Conservative former prime ministers Theresa May and Sir John Major and the former Conservative Party leader Lord Howard.
Sir John and fellow former PM Tony Blair described the legislation as “shaming” and said it “questions the very integrity of our nation”.
Mr Johnson, with a large Commons majority, should win an expected vote of the bill’s principles during the second reading on Monday.
However, Commons justice committee chairman Sir Bob Neill is laying an amendment, which he said would impose a “parliamentary lock” on any changes to the Brexit deal.
Labour minister Rachel Reeves said the party will vote against the government’s bill if it still contains clauses overriding the withdrawal agreement.
Additional reporting by Press Association
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