Boris Johnson is braced for weeks of criticism from MPs and peers as his internal market bill receives sustained scrutiny for the first time in Parliament from Monday.
The list of high-level figures opposed to the legislation, which overrides part of the Brexit withdrawal agreement, was joined on Sunday by Tony Blair and John Major, two former prime ministers.
Other notable figures who have spoken out against it include former leader Michael Howard and one-time chancellor Norman Lamont, who are both senior Conservative Brexiters.
The bill has also caused anger in the US where House speaker Nancy Pelosi has warned there is “absolutely no chance” of a US-UK trade deal passing through Congress if the legislation is approved.
Mr Blair and Sir John, former Labour and Conservative leaders respectively, said the proposal to renege on parts of last year’s Brexit divorce deal threatened “the very integrity of our nation”.
The internal market bill includes clauses that override the Northern Ireland protocol, the part of the withdrawal agreement designed to prevent a hard border returning to the island of Ireland.
Mr Johnson, the UK prime minister, said the clauses were necessary to prevent the EU from imposing a “full-scale trade border down the Irish Sea”.
The legislation would give ministers powers to modify or disapply rules relating to state aid and to the movement of goods between Britain and Northern Ireland if there was no UK-EU trade deal by January 1.
The government is braced for a rebellion by a significant handful of backbench Tory MPs as well as stormy sessions in the House of Lords later in the month.
Opposition from at least 20 Tory MPs is expected to coalesce around an amendment by Sir Bob Neill, chair of the justice select committee, although that may not be voted on for another week.
Even if that rebellion is headed off it could pave the way for a historic defeat in the House of Lords later on.
Monday will see the second reading of the bill, with a single vote that the government is expected to win easily, although some Tory rebels are expected to abstain.
Geoffrey Cox, the former attorney-general, has joined the ranks of the rebels in an article in Monday’s Times newspaper, which said the legislation risked the “standing and reputation of Britain in the world” by breaching international law.
Last week some Labour figures were privately debating whether or not to accept the legislation for fear of appearing to be somehow trying to block Brexit. The mantra of Keir Starmer, the new leader, is “Get Brexit Done” in order to focus on the Covid-19 pandemic.
But Rachel Reeves, the shadow Cabinet Office secretary, told the BBC that Labour would vote against the legislation because it “deliberately and consciously” breaks international law.
Tuesday and Wednesday will see votes on amendments around the UK internal market, with the Scottish National Party and Plaid Cymru expected to oppose “mutual recognition” principles that could stop Scotland and Wales preventing the sale of products, even if they cannot be produced in those countries.
Mr Blair and Sir John accused the government of “embarrassing the UK” with legislation that was “irresponsible, wrong in principle and dangerous in practice” and said that respecting treaty obligations was “just as important” as domestic law.
Robert Buckland, justice secretary, argued on Sunday that the government was still “absolutely committed to the rule of law” and had drawn up the provisions in the legislation as a fallback in the event that no EU trade deal was struck.
Michel Barnier, the EU’s chief negotiator, meanwhile dismissed claims by Mr Johnson that the protocol on Northern Ireland was a threat to the integrity of the UK.
“We agreed this delicate compromise with Boris Johnson and his government in order to protect peace and stability on the island of Ireland,” he said. “We could not have been clearer about the consequences of Brexit.”
Mr Barnier separately denied that the EU was threatening to withhold “third-country” status from the UK to make it harder for food deliveries from Great Britain to Northern Ireland.
“The EU is not refusing to list the UK as a third country for food imports,” he said on Twitter. “To be listed, we need to know in full what a country’s rules are, including for imports. The same objective process applies to all listed countries.”