CIA warns Barack Obama that British terrorists are the biggest threat to the US

Barack Obama has been warned by the CIA that British Islamist extremists are the greatest threat to US homeland security. American spy chiefs have told the President that the CIA has launched a vast spying operation in the UK to prevent a repeat of the 9/11 attacks being launched from Britain.

They believe that a British-born Pakistani extremist entering the US under the visa waiver programme is the most likely source of another terrorist spectacular on American soil.

Intelligence briefings for Mr Obama have detailed a dramatic escalation in American espionage in Britain, where the CIA has recruited record numbers of informants in the Pakistani community to monitor the 2,000 terrorist suspects identified by MI5, the British security service.

A British intelligence source revealed that a staggering four out of 10 CIA operations designed to thwart direct attacks on the US are now conducted against targets in Britain.