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.
And a former CIA officer who has advised Mr Obama told The Sunday Telegraph that the CIA has stepped up its efforts in the last month after the Mumbai massacre laid bare the threat from Lashkar-e-Taiba, the militant group behind the attacks, which has an extensive web of supporters in the UK.
The CIA has already spent 18 months developing a network of agents in Britain to combat al-Qaeda, unprecedented in size within the borders of such a close ally, according to intelligence sources in both London and Washington.
Bruce Riedel, a former CIA officer who has advised Mr Obama, told The Sunday Telegraph: "The British Pakistani community is recognised as probably al-Qaeda's best mechanism for launching an attack against North America.
"The American security establishment believes that danger continues and there's very intimate cooperation between our security services to monitor that." Mr Riedel, who served three presidents as a Middle East expert on the White House National Security Council, added: "President Obama's national security team are well aware that this is a serious threat."
The British official said: "The Americans run their own assets in the Pakistani community; they get their own intelligence. There's close cooperation with MI5 but they don't tell us the names of all their sources.
"Around 40 per cent of CIA activity on homeland threats is now in the UK. This is quite unprecedented."
Explaining the increase in CIA activity over the past month, Mr Riedel added: "In the aftermath of the Mumbai attack the US and the UK intelligence services now have to regard Lashkar-e-Taiba as just as serious a threat to both of our countries as al-Qaeda. They have a much more extensive base among Pakistani Diaspora communities in the UK than al–Qaeda."
Information gleaned by CIA spies in Britain has already helped thwart several terrorist attacks in the UK and was instrumental in locating Rashid Rauf, a British-born al-Qaeda operative implicated in a plot to explode airliners over the Atlantic, who was tracked down and killed in a US missile strike in November.
----------------------------What about Canada?