Former prime ministers Tony Blair and John Major on Sunday said Britain must drop a "shocking" plan to pass legislation that breaks its divorce treaty with the European Union, in a breach of international law.
The British government said explicitly last week that it plans to break international law by breaching parts of the Withdrawal Agreement treaty that it signed in January, when it formally left the EU.
"What is being proposed now is shocking," Major and Blair, who were adversaries in the 1990s as Conservative and Labour leaders, wrote in a joint letter published by the Sunday Times newspaper.
"How can it be compatible with the codes of conduct that bind ministers, law officers and civil servants deliberately to break treaty obligations?"
Theresa May, current Prime Minister Boris Johnson's predecessor, has also questioned whether international partners would be able to trust Britain in future.
Johnson says the new legislation is needed to clarify the Northern Ireland protocol element of the Brexit deal, to protect free trade between the four constituent nations of the United Kingdom.
EU chief negotiator Michel Barnier said on Sunday that the Withdrawal Agreement on Northern Ireland "is not a threat to the integrity of the UK", and had been agreed by the two sides to protect peace on the island of Ireland.
"We could not have been clearer about the consequences of Brexit," Barnier said on Twitter.
Earlier on Sunday, British Justice Secretary Robert Buckland told Sky News the legislation is "Brexit-critical" and must be passed by parliament, which is due to debate the bill on Monday.
Keir Starmer, leader of the opposition Labour Party, described the legislation as wrong on Sunday.
"We have broken the trust of our international partners," Starmer wrote in the Sunday Telegraph newspaper, adding that his party would oppose the bill in parliament unless changes were made.
European lawmakers have warned they would not approve any new trade deal unless the withdrawal agreement was fully implemented, while there is also talk of possible legal action.
"The reputation of the UK ... as a trusted negotiating partner on important issues like this is being damaged in a very serious way," Irish foreign minister Simon Coveney told the BBC on Sunday.