May, now a Tory backbencher, gave a stinging analysis of the internal market bill which, if enshrined into law, will override key aspects of the Withdrawal Agreement.
Speaking in the Commons, May claimed the controversial bill would cause “untold damage” to the United Kingdom.
“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,” she said.
“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.”
The former Tory prime minister, who helped negotiate chunks of the Withdrawal Agreement, pleaded with the government to recognise the UK as 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 added.
“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.”
May said the treaty already had provisions for an arbitration process and that Johnson’s controversial additions had “no place in this bill”.
She said: “I cannot emphasise how concerned I am that a Conservative government is willing to go back on its word, to break an international agreement signed in good faith and to break international law.”
Attacking the Neill amendment that requires the government to seek support of MPs before invoking controversial elements of the internal market bill, May said: “Frankly, my view is to the outside world it makes no difference as to whether a decision to break international law is taken by a minister or by this parliament – it is still a decision to break international law.
“This can only weaken the UK in the eyes of the world.
“One of the great strengths we have as a country is our commitment to the rule of law and this will have been damaged.
“Our reputation as a country that stands by its word will have been tarnished.
“And the willingness of other countries to trust the United Kingdom and its values will have been reduced.
“So much for global Britain.”
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.