Britain's top intelligence official is in a coma in a London hospital after collapsing at home, police and his office said on Friday.

Alex Allan, 56, chairman of the government's Joint Intelligence Committee (JIC), the body which oversees the work of the security services at home and abroad, fell ill on Monday.

Media reports said he had undergone toxicology tests to check whether he was deliberately poisoned, although police said there was no suspicion of foul play.

"We were made aware of a man in his late-50s taken to a west London hospital after having been taken ill," the Metropolitan Police said. "This is being treated as non-suspicious."

A Cabinet Office spokeswoman confirmed Allan was in a coma after falling unconscious. She gave no more details about the nature of his illness.

"We can't discuss his condition because it is a private matter," she said.

Police became involved because two officers happened to be in the hospital where Allan was taken and started making inquiries, according to a report in the Times.

Allan had complained to colleagues last week that he wasn't feeling well, the report added.

"There were no indications that anything was wrong apart from him saying that he was not feeling quite up to scratch," one unnamed colleague was quoted as saying.

As head of the JIC, Allan gives ministers assessments of security, defence and foreign threats to Britain's national interests.

His committee scrutinises the work of the intelligence agencies MI5, MI6 and the security monitoring centre GCHQ. He has direct access to Prime Minister Gordon Brown.

He was appointed last November, moving from his position as top civil servant at the Ministry of Justice. His artist wife Katie Clemson died of cancer last year.

The JIC was thrust into the public spotlight before the invasion of Iraq when one of its intelligence reports was used by the government to help make the case for going to war.