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.
I'm sure that some will start screaming about racial profiling right away.