The Government's top intelligence adviser is seriously ill in hospital.

Alex Allan, who heads the Joint Intelligence Committee (JIC), fell ill on Monday.

Whitehall sources said it was unclear why he collapsed but they did not believe the cause was "foul play".

Scotland Yard said the incident was being treated as "non-suspicious".

A Cabinet Office spokeswoman said: "We can confirm that Alex Allan was taken ill on Monday and transferred to a London hospital.

"We cannot discuss his condition as it is a private matter."

A spokesman for the Metropolitan Police said: "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."

The 57-year-old's wife, artist Katie Clemson, died from cancer last November.

Mr Allan reported feeling unwell towards the end of last week and remains in a coma while the hospital conducts tests.

The JIC is part of the Cabinet Office and responsible for briefing the Prime Minister on a weekly basis. Mr Allan was appointed to the position last November. He was previously permanent secretary at the Ministry of Justice.