British former Prime Minister David Cameron has also spoken out against Boris Johnson's controversial bill, which undermines parts of the Brexit withdrawal agreement.
This makes him the fifth former prime minister to fiercely criticize Johnson.
"Passing a law and then breaking an international obligation is the very last thing you should consider. It's really a last resort," Cameron told reporters Monday.
"So I have my doubts about this."
Former Prime Ministers Tony Blair and John Major already turned against the plan on Sunday, and Theresa May and Gordon Brown have also said they are critical of it.
The UK government presented the bill on Wednesday.
The proposal explains how the British intend to trade within the UK after Brexit and, if passed, would give London the power to overrule parts of the so-called Northern Ireland Protocol.
This protocol was drawn up last year by the EU and the UK to avoid a hard border on the Irish island.
At the time, Johnson called the deal "very good" and "oven-ready".
With the new bill, Johnson returns to those words.
The British Prime Minister told the British House of Commons on Wednesday that the proposal is a "safety net for the UK against extreme or irrational interpretations" of the Northern Ireland protocol.
The bill would be necessary to avoid "harmful" import tariffs on goods going to Northern Ireland from Great Britain.
The government also admitted last week that the proposal would violate international law in a "very specific and limited" way.