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Downing Street's response is a classically formal "thanks, but no thanks". A stiff brush-off in riposte to the US president's digital tirade, which was extraordinary even by his standards.

With the current prime minister almost out of the door, and the UK ambassador in Washington leaving too, the remarks are unlikely to change much directly, and this allows Number 10 to try to shrug off the criticism.

Less officially, though, there is real frustration. One senior Tory warned that "we cannot bow down to this form of lunacy" where the leader of another country tries to use online swagger to seek revenge on one of the UK's diplomats - not least from one of our most important allies.

The government, of course, has become rather used to managing the maverick. But this episode makes the choice of the next ambassador to the United States a controversy waiting to happen.

And it brings up a crucial foreign policy question for the two men vying to be prime minister, which they could be asked in the leadership debate on Tuesday evening - do they try to tackle Donald Trump, tame him, or merely tolerate him?

Looking the other way, would the next prime minister be blind to the damage that's already being done.