So far, British intelligence is denying that their chief, Alex Allan, who chairs the Joint Intelligence Committee, is the victim of an assassination attempt after he was found in his home in a coma. But given what happened to Alexander Litvinenko, a Russian agent poisoned by radioactive tea, it is natural to wonder aloud if this too, could be some kind of attack:

The Metropolitan Police rushed officers to his bedside as soon as Mr Allan was taken to hospital after being alerted that he was a senior figure from the world of intelligence.

But their inquiries produced no evidence of foul-play or any suggestions that he might have tried to take his own life. In a statement Scotland Yard ruled it was a "non suspicious" incident.

Sources said it appeared that Mr Allan had fallen ill at home at some point at the weekend and lay undiscovered until Monday afternoon.

"It's a mystery. He's suffered some fairly traumatic and sudden illness that hasn't been helped by the fact that he lay undiscovered for some time," a friend said.

Friends who saw him last week said he appeared "his usual ebullient self" although he later complained of feeling unwell. He had discussed with colleagues his plans for the summer.

It also appeared that Mr Allan, who is well known in Whitehall as a fan of the 1970s American cult group the Grateful Dead, visited a site dedicated to the band on Saturday morning.