Former British prime minister David Cameron joined other ex-leaders of the country to express concern at Boris Johnson's plan to break international law by overriding parts of the Brexit divorce treaty with the European Union.
"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 an absolute final resort," Cameron, from Johnson's Conservative Party, told reporters on Monday.
"So I do have misgivings about what is being proposed."
Former prime ministers Tony Blair and John Major said on Sunday that Britain must drop its "shocking" plan.
However, Cameron did not go that far, saying a proposed bill that lawmakers will vote on later on Monday - and which the government has said would breach international law - had to be seen in the context of tough trade talks with the EU.
"So far what's happened is the government has proposed a law that it might pass or might not pass, might use or might not use, depending on whether certain circumstances do or do not appear," he said.
"The bigger picture here is we are in a vital negotiation with the European Union to get a deal and I think we have to keep that context, that big prize in mind."