UK intelligence chief Alex Allan is in coma. He was found unconscious at home in West London and rushed to hospital earlier in the week. But doctors have not been able to detect what he is ailing from.
There were initial fears that he might have been poisoned, leading to comparisons with Alexander Litvinenko, the former officer of the Russian state security service who died after being poisoned by the radioactive metal polonium-210.
However, government sources said poisoning was unlikely due to Allan's senior position, and police said they did not consider his illness suspicious. Still they ran toxicology tests on him.
A government source said he was "so high-profile that he would be a very unlikely target for attack. He is a civil servant and he doesn't have enemies. There is no reason for him to be targeted by anyone."
56-year-old Allan was appointed chairman of Whitehall's joint intelligence committee in November last year after a career as one of Britain's leading civil servants. His job involves analysis of MI6, US and GCHQ data rather than intelligence-gathering.
Last year he was criticised for publishing personal details on his website that showed him to be far removed from the traditional image of a grey civil servant. He is a follower of American rock group The Grateful Dead. He is also a keen cyclist.
Known as an outgoing and colourful character, he once windsurfed down the Thames wearing a bowler hat and suit.
One colleague said: "He is a very charismatic person and there were no indications that anything was wrong apart from him saying that he was not feeling quite up to scratch."