Britain's top spy is unconscious and seriously ill in hospital, it has emerged.

Alex Allan, 57, collapsed at his home earlier this week, foul play is not suspected.

BBC Security Correspondent Gordon Corera said that "there is no police involvement" or "concern over the reason for his condition".

The Metropolitan Police said in a statement: "We were made aware of a man in his late 50s who was taken to a London hospital after being taken ill at his home address.

"He remains in a serious condition. This is being treated as non-suspicious."

A Sky News source said "toxicology tests have been conducted" but "that is normal when people are rushed to hospital in an unconscious state".

Earlier reports had speculated that the spy had been poisoned but this has been denied by Government sources.

Mr Allan began his Whitehall career in 1973 and was a civil servant who worked mainly on the Treasury.

He become a principal private secretary to both John Major and Tony Blair when they were in office in 10 Downing Street. He was also High Commissioner to Australia in 1997.

In 1999 he become 'e-envoy' for the Government before moving back to Australia a year later. He returned to the UK in 2004.

Downing Street announced Mr Allan's appointment as the Chairman of the Joint Intelligence Committee and Head of Intelligence Assessment last year.

Mr Allan's artist wife, Katie Clemson, died from cancer last November.

(DS)