IF anyone had any doubts that Donald Trump is a law unto himself, then his extraordinary comments on Theresa May’s carefully calibrated Brexit plan would have banished them.

One can only imagine, during the dessert course at the Blenheim Palace black-tie dinner on Thursday night, as word got round about the Donald’s incendiary Sun interview, that the Prime Minister’s spoon slipped from her mouth as her jaw hit the dining table.

That his assertion - that the softish Chequers Plan would “probably kill” any chance of a post-Brexit UK-US Trade deal - was an off-the-cuff remark is nonsense; naive the American President ain’t.

This was a carefully-targeted Exocet, guided by the pro-Brexit newspaper, to do maximum damage to Mrs May’s compromise plan. It had Rupert Murdoch’s fingerprints all over it.

There was outrage across the political divide and even from Government ministers at Mr Trump’s “insulting” and “rude” outburst.

But the Donald doesn’t give a hoot about protocol. He plays by his own rules and is only interested in his own voice and his own opinions. So he is publicly discourteous to his oldest and staunchest ally, so what. He knows weakness when he sees it and exploits it to the full.

To add insult to injury, which no doubt in the Trump playbook is a win-win outcome, the President suggests the Tory leader’s nemesis, Boris Johnson, would make a great Prime Minister.

Sir Alan Duncan, the Foreign Office Minister, drew the short straw this morning and had to go on the airwaves to limit the damage; mission impossible.

The best he could do was to describe the President as “very unconventional” and a “controversialist”. Understatement.

As Mrs May prepares for a joint press conference, she will be screaming inside. And Mr Trump will be enjoying every moment.