Amal Clooney has quit as Britain's special envoy on media freedom in protest at the Government's "lamentable" decision to override the Brexit divorce settlement, in breach of international law.
In a letter to Foreign Secretary Dominic Raab, the prominent human rights lawyer said she had no alternative but to resign after he made clear the Government would change its position.
"Although the Government has suggested that the violation of international law would be 'specific and limited', 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," she wrote.
"Out of respect for the professional working relationship I have developed with you and your senior colleagues working on human rights, I deferred writing this letter until I had had a chance to discuss this matter with you directly.
"But having now done so and received no assurance that any change of position is imminent, I have no alternative but to resign from my position."
Ms Clooney, a British-Lebanese barrister, was originally appointed to the role of special envoy by the then foreign secretary Jeremy Hunt in 2019.
In her letter to Mr Raab, she said she was "disappointed" to have to give up the position having always been proud of the UK's reputation as "a champion of the international legal order and of the culture of fair play".
"However, very sadly, it has now 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 said.
Her decision to quit follows the resignations of two other senior lawyers – the advocate general for Scotland, Lord Keen of Elie, and the head of the Government Legal Department Sir Jonathan Jones.
It will add to the pressure on Boris Johnson who has been facing calls to drop measures in the UK Internal Market Bill enabling ministers to override provisions in the Brexit Withdrawal Agreement relating to Northern Ireland.
Mr Johnson appeared to have headed off a looming Commons revolt through a deal with Tory rebels in which he agreed there would have to be a vote by MPs before ministers could activate the powers.
However, it is not clear whether the move will be enough to placate all the critics of the Bill who voiced deep concern that the Government would even consider going back on its international treaty obligations.
Former Conservative party leader Lord Howard of Lympne has already said the change "isn't enough" for him.
Even if the Bill gets through the Commons unscathed next week, there are likely to be further attempts to amend the legislation when it gets to the House of Lords.