David Cameron on Monday became the fifth former UK Prime Minister to speak out against the new controversial market bill that will override parts of the Brexit Withdrawal Agreement, it was reported.
The Internal Market Bill, which will be formally debated in the House of Commons for the first time on Monday, addresses the Northern Ireland Protocol - the part of the Brexit Withdrawal Agreement designed to prevent a hard border returning to the island of Ireland, the BBC reported.
The government is expected to win a vote in the Commons later on the next stage of the bill - which is expected at around 10 p.m. on Monday - but it is expected to face more difficulties in later stages, especially when the legislation heads for debate in the Lords.
Addressing the media, Cameron, who called the European Union (EU) referendum in 2016 when he was Prime Minister, said he had "misgivings about what is being proposed".
"Passing an act of Parliament and then going on to break an international treaty obligation is the very, very last thing you should contemplate. It should be the absolute final resort."
Cameron said the "bigger picture" was about trying to get a trade deal with the EU, urging the government to "keep that context (and) that big prize in mind."
The comments follow stronger criticism by the four other former Prime Ministers of the UK.
Incumbent Prime minister Boris Johnson's predecessor Theresa May, who still sits as an MP in the Commons, said breaking international law would damage "trust" in the UK, while Gordon Brown said it would be akin to "self-harm" for the country.
John Major and Tony Blair, who were both in office during key periods of the Northern Ireland peace process, wrote a joint article in the Sunday Times accusing Johnson of "embarrassing" the UK and urging MPs to reject the "shameful" attempt to override parts of the withdrawal agreement.
The UK left the EU on January 31, having negotiated and signed the withdrawal agreement, which is now an international treaty, with the bloc, the BBC reported.
The two sides are now in the closing weeks of negotiations for a post-Brexit trade deal before the transition period ends on December 31, with informal talks taking place in Brussels this week.
A key part of the withdrawal agreement was the Northern Ireland Protocol, designed to prevent a hard border returning to the island of Ireland.
The Internal Market Bill proposed by the Johnson-led government would override that part of that agreement when it came to goods and would allow the UK to modify or re-interpret "state aid" rules on subsidies for firms in Northern Ireland, in the event of the two sides not agreeing a future trade deal.
Last week, Northern Ireland Secretary Brandon Lewis said the bill would "break international law" in a "specific and limited way", leading to swathes of criticism from all sides of the political spectrum.