Theresa May has vowed to rebel over the Brexit bill that ministers admitted will break international law, warning it “puts the future of the United Kingdom at risk”.
In a fiery Commons speech, the former prime minister said Boris Johnson’s partial concession – a parliamentary lock on using the legislation’s powers – would still leave the UK flouting “the rule of law”.
Ms May accused her successor of failing to understand the implications of the Northern Ireland protocol he had signed last year – something No 10 has denied.
And she told MPs: “The government is acting recklessly and irresponsibly with no thought for the long-term impact on the standing of the United Kingdom in the world.
“This will lead to untold damage to the United Kingdom's reputation, it puts the future of the United Kingdom at risk and, as a result, with regret, I have to tell the minister I cannot support this bill.”
Ms May pleaded with the government to recognise that the UK “is a country that upholds the rule of law – it is one of the things that makes us great”.
“Yet we are being asked to tear up that principle and throw away that value,” she warned, in debate on the Internal Market Bill
“And why? I can only say, on the face of it, it’s because the government didn’t really understand what it was signing up to when it signed the withdrawal agreement.”
The government was “acting recklessly and irresponsibly”, putting the status of Northern Ireland as part of the UK in jeopardy.
The legislation has provoked uproar in the EU and the USA, by paving the way to override legal agreements with Brussels on using state aid and requiring customs checks on goods crossing the Irish Sea.
The prime minister was accused of bringing a no-deal Brexit significantly closer and of undermining both the still-fragile peace in Northern Ireland and the UK’s worldwide reputation.
Downing Street has argued the measures are a fallback option, should the trade talks fail, and insisted it remains committed to the withdrawal deal, an international treaty.
Mr Johnson is also confident he has seen off a major Conservative revolt after agreeing that MPs must give approval before the powers in the bill can be enacted.
However, it is near-certain to be amended in the House of Lords, creating a race against time for it to be in place before the post-Brexit transition period ends on 31 December.
Robin Walker, a Brexit minister, hit back at the former prime minister, saying: “As [Mrs May] knows well, the withdrawal agreement was negotiated by the UK and the EU, and agreed with a view that certain elements would be resolved by the joint committee.
“I think there was a reasonable expectation on both sides that the joint committee would have made more progress on those issues and we have heard unfortunately some harmful interpretations suggested over the last few months.
“The point of these government clauses is to ensure that we can rule those out and put in place the appropriate legal default.”