Spy chief in poison mystery

Britain's leading spy was today fighting for his life after he mysteriously fell into a coma.

Alex Allan, the head of MI5, MI6 and GCHQ, was found unconscious at his west London home on Monday and taken to hospital.

The 56-year-old chairman of the Joint Intelligence Committee's condition deteriorated after he complained of feeling unwell towards the end of last week.

Friends say he is a keen athlete who is extremely fit and doctors are trying to find out whether he has contracted pneumonia. Toxicology experts were also carrying out tests.

Gordon Brown was immediately informed of Mr Allan's condition, which was described as critical, because of his role at the top of the intelligence services.

The civil servant had direct access to the Prime Minister and gave him regular briefings about the terrorist threat to Britain.

His unexpected coma has raised fears of an assasination attempt which coincide with a warning from intelligence sources today that Russia is now the third biggest threat to national security after al Qaeda and Iranian nuclear proliferation.

Sources say the three main Russian spy agencies have "flooded" Britain with agents dedicated to military and industrial espionage.

Former KGB spy Alexander Litvinenko was murdered in London in November and Russian agents have been accused of poisoning his drink with radioactive polonium.

The members of the Joint Intelligence Committee Mr Allan chairs include the heads of the UK's three intelligence agencies, MI5, MI6 and GCHQ. The committee is responsible for giving ministers assessments of the country's security.

Security expert Chris Dobson said of the sudden illness: "Alex Allan's illness raises suspicions of foul play simply because of his job. He oversees and coordinates every aspect of our intelligence community.

"He is therefore a prime target for an assassination attempt by Britain's enemies. The nature of his sudden illness, if it is an assassination attempt, points towards the FSB, successors of Russia's KGB. They are acknowledged masters of assassination by poison," he told The Sun.

"Al Qaeda is another suspect. They would see his death as a great victory, fulfiling Osama Bin Laden's threat to strike at the heart of the 'infidel' enemy. What better target than the man whose job is dedicated to wiping them out?"

Despite the fears security sources say they are "as certain as we can be at this stage" that Mr Allan was not the target of a sinister attack by Bin Laden's operatives in Europe.

A Whitehall colleague who saw him recently said: "He was fit, he was in cracking form when I saw him. He was very upbeat. Although his wife's death was a tragedy, it had happened some time ago."

Mr Allan's 58-year-old Australian wife Katie Clemson died from cancer in November.

The senior civil servant had also been asked to explain why top secret documents were found on a train and handed to the BBC last month. Despite these concerns, colleagues insisted he was upbeat and that his job was safe.

The Whitehall source added: "It's a tragedy because he was such a lovely man. I can't believe it is anything other than natural causes."

Intelligence sources added there would be a huge challenge replacing Mr Allan if that were necessary.

His role as chairman of the Joint Intelligence Committee was changed significantly in the wake of the Butler inquiry into the intelligence failings in the lead-up to the Iraq war.

The report specified that the Committee chairman should be someone of high standing within Whitehall but who had no further career ambitions.

"It is extremely difficult to find someone who is both extremely talented and willing to accept a job knowing that it will be their last," the source said.

Mr Allan's career includes a spell as private secretary to former prime ministers John Major and Tony Blair.