He was found by Dominique Salm, a painter who rents an artist's studio in his west London home.

According to neighbours she found him slumped unconscious with "blood everywhere".

Doctors have run a battery of test to try and establish why the 57-year-old Chairman of the Joint Intelligence Committee collapsed at his home on Monday.

Government sources insist there is no evidence of suspicious circumstances, but Miss Salm's account will add to speculation that Mr Allan may have been targeted by a foreign spy.

The painter's mother Sally Ann Salm told the Mail on Sunday: "Some very serious people have asked Dominique not to comment on any of this until it has been fully investigated and she is doing just that. So am I."

The Counter Terrorism Command has investigated but is so far treating the illness as non-suspicious.

A police source said: "Because he is a person of significance, investigators made sure nothing untoward had occurred. But it was quickly established there was a medical reason for how he was found."

Whitehall sources are blaming the collapse on pneumonia, and experts agree that prolonged coughing can bring up blood.

Dr Keith Prowse, chairman of the British Lung Foundation, said: "If it's pneumonia it is likely to be secondary to something else. And a significant quantity of blood is more likely to come from the gut than the chest.

"Pneumonia is something that can strike very quickly, however."

Mr Allan has been described as "ebullient and out-going" and once windsurfed down the Thames in a suit and bowler hat to avoid a transport strike.

He was private secretary to both John Major and Tony Blair and was previously permanent secretary at the Ministry of Justice and High Commissioner for Australia.

He was appointed to be chairman of the Joint Intelligence Committee (JIC) by Gordon Brown a year ago and took up his job in the autumn.

It is Mr Allan's job to assess the intelligence put forward by MI5, MI6, GCHQ, military intelligence and foreign intelligence sources.

He reports to the permanent secretary for intelligence, security and resilience who in turn reports to the Prime Minister.

Government sources suggest Mr Allan was "too high profile" to be a target for foreign intelligence agencies but others have suggested this is exactly what could make him vulnerable.

Miss Salm has been using a studio at Mr Allan's home since the death of his artist wife, Katie Clemson, in November last year.