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.
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.
But some US intelligence officers are irritated that valuable manpower and resources have been diverted to the UK. One former intelligence officer who does contract work for the CIA dismissed Britain as a “swamp” of jihadis.
Jonathan Evans, the director general of MI5, admitted in January that the Security Service alone does not have the resources to maintain surveillance on all its targets. “We don’t have anything approaching comprehensive coverage,” he said.
The dramatic escalation in CIA activity in the UK followed the exposure in August 2006 of Operation Overt, the alleged airline bomb plot.
The British intelligence official revealed that CIA chiefs sent more resources to the UK because they were not prepared to see American citizens die as a result of MI5’s inability to keep tabs on all suspects, even though the Security Service successfully uncovered the plot.
MI5 manpower will have doubled to 4,100 by 2011 but many in the US intelligence community do not think that is enough.
For their part, some British officials are queasy that information obtained by the CIA from British Pakistanis was used to help target Mr Rauf, a British citizen, whom they would have preferred to capture and bring to trial.
Sensitivities over the intelligence arrangement formed a key part of briefings given to Mr Obama, since they are central to what is often called “the most special part of the special relationship” and could complicate his dealings with Gordon Brown.
Tensions in transatlantic intelligence relations which were laid bare last week during the High Court battle over Binyam Mohamed, the British resident held in Guanatanamo Bay. British judges wanted to publish details of the torture administered to Mr Mohamed, an Ethiopian national, in US custody. But key paragraphs were blacked out after American officials threatened it could damage intelligence sharing between the two countries.
Intelligence experts said that a trusting intelligence relationship, in which one country does not publish intelligence data obtained by the other, is vital to both countries’ national security.
Patrick Mercer, chairman of the House of Commons counter-terrorism sub-committee, said: “The special relationship is a huge benefit to us. It clearly works to our advantage and helps keep the people of the UK and the US safe.
“There is no doubt that a great deal of valuable intelligence vital to British national security is procured by American agents from British sources.”
Mr Riedel added: “The partnership between the two intelligence communities is dynamic; it is one of great intimacy. We overuse the term special relationship, but this is an extraordinarily special relationship.
“Since September 11 the philosophy on both sides has been to err on the side of telling each other more rather than less. It is in everyone’s interests that that continues.”