Amal Clooney has quit her role as the UK’s envoy on press freedom “in dismay” at the government’s willingness to break international law over Brexit.
The human rights lawyer said it was “lamentable” for Boris Johnson to be contemplating overriding the Brexit agreement he signed last year.
She could not tell others to honour legal obligations when the UK “declares it does not intend to do so itself”.
The PM says he does not want to use the powers in the Internal Markets Bill.
But he says the legislation is necessary to give the government the power to protect the UK and, particularly Northern Ireland, if trade talks fail and the EU acts “unreasonably”.
In her resignation letter, Ms Clooney, who is married to Hollywood actor George Clooney, said she had accepted the job last year because of the UK’s historic role in upholding the international legal order.
But she said the government’s attempts to pass the Internal Market Bill, which passed its first hurdle in the Commons last week, made her position “untenable”.
She said she had failed to get the reassurances she was seeking following a conversation with foreign secretary Dominic Raab.
“Out of respect for the personal working relationship I have with you and your senior colleagues working on human rights, I deferred writing this letter until I had a chance to discuss this matter with you directly.
“But having done so and received no assurance that any change of position is imminent, I have no alternative to resign my position.”
She added: “It is lamentable for the UK to be speaking of its intention to violate an international treaty signed by the Prime Minister less than a year ago.
“It has become untenable for me, as Special Envoy, to urge other states to respect and enforce international obligations while the UK declares that it does not intend to do so itself.”
She was the deputy chair of the high-level panel of legal experts which works with the UK and Canadian governments on their campaign to promote media freedom around the world.