Reuters reports that former British prime minister David Cameron expressed 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 told.

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.