The treatment of Kim Darroch is beyond far-fetched. But here we are embroiled in the maddest thriller of all

During an hour in front of the Foreign Affairs Committee Sir Simon McDonald, the UK’s most senior diplomat, made clear that the trust lost during the spat will not easily be regained

“Diplomatic language” is the polite term for the words used by people who won’t say much. The phrase “diplomatic language” is itself diplomatic language. Diplomats’ words are their currency, and they are investors, not spenders.

So when the UK’s most senior diplomat, Sir Simon McDonald, is sitting in a grand oak-panelled committee room in the Palace of Westminster, and the following phrases exit his damp lips, there is a sense that history is occurring.

“The leaker is guilty of the worst breach of trust in our service in my career,” he said, matter-of-factly but plainly aware of the gravity of the situation. “We will pursue the culprit with all means at our disposal. It is very important that the person is caught.”

He was addressing a special session of the Foreign Affairs Committee, arranged, to hear evidence on the leaking of diplomatic briefings by the British ambassador in Washington DC, Sir Kim Darroch, that culminated on Tuesday in President Trump announcing, on Twitter of course, that the ambassador had been frozen out by the president.

The room was hot. The air was dry. The committee’s members looked upon McDonald with furious intensity. Above their heads, a grand portrait of Neville Chamberlain did the same.

It was due to be a dramatic enough affair, and then, an hour before it began, Sir Kim resigned. For an ambassador to be bullied out of his job by the president of his host country is serious enough. For the countries in question to be the UK and the United States, is extraordinary.

“Is there another time when a friendly government has refused to cooperate with one of Her Majesty’s envoys?” Tom Tugendhat, who chairs the committee, asked.

“None,” came the reply.

The short word bounced off the walls. It hung in the air like a jury foreman delivering the verdict in the climactic moment of a John Grisham movie adaptation. Chamberlain’s eyes widened.

The last time anything like this had happened was long before his time. “The last time we had difficulties with the United States was 1856,” Sir Simon said. “When the incumbent was accused of recruiting Americans to fight on the British side in the Crimean War.”

He had spoken to Kim Darroch. “He had a difficult night, considering what to do,” he said. “He thought about the pressure on his family. He considered that for as long as he was in Washington, he would remain a target.”

“A target”. The British ambassador, in Washington DC. An idea from a place where Graham Greene or John le Carré would never bother to go. It is too far-fetched. Yet here it was, all playing its way out, right here in reality, the maddest thriller of them all.

Priti Patel, Bob Seeley and various others did their level best at amateur detective work. Who sees communications? Do the computers used have a “forward” button, Mr Tugendhat wanted to know. All computers have a forward button, he was told.

“Some of the military systems don’t,” he said. Mr Tugendhat is a very senior figure in the Tory army boys clique. It is unclear whether anyone has spent more than 10 consecutive seconds in his company without his raising the fact he had once been in the armed forces. No trip to the House of Commons canteen can ever pass without some reference to some mess hall or other. As always, in politics, self-regard is everything. Mr Tugendhat looks in the mirror and sees the Duke of Wellington. It is a pity that everybody else looks at Mr Tugendhat and sees Uncle Albert, though the greater pity is that he will never know.

Who could have seen the files? Who could they have sent them to? Could they have taken a photograph of the screen? All these questions were entertained by Sir Simon, but none of them mattered. It was the kind of conversation that belonged in a WhatsApp group after last night’s Line of Duty.

The Foreign and Commonwealth Office’s primary security protocols are not done on its IT systems. They are done at interview stage. It is a matter of trust. The systems are simply not in place for it to be broken in this way.

Sir Simon was gone within an hour, on his way to repeat most of his words to an emergency meeting of all Foreign Office staff. How that trust is re-established is a question for which he did not appear to have any answer at all.