Two of the terror suspects being questioned over the alleged plot to blow up transatlantic airliners are being investigated for links with the 9/11 attacks on the United States.
The two suspects may have had contacts with a man accused of helping to organise the al-Qa'ida attacks on America in 2001.
The possible connection came as British and international intelligence services continued to investigate the backgrounds of the 23 British men and women being held in custody on suspicion of plotting to destroy up to 10 US-bound aircraft.
As part of the inquiry British and German intelligence are tracing possible contacts between two of the suspects and a man named Said Bahaji, who is wanted for allegedly being part of an al-Qa'ida cell that included Mohammed Atta, the lead pilot of the two planes that rammed into the World Trade Centre. Mr Bahaji was based in Germany prior to the strikes on New York and Washington, which he is accused of helping organise, but is believed to have fled to Pakistan.
Uncorroborated reports state that the suspected bombers had contacted Mr Bahaji through his wife who still lives in Hamburg. It is believed that calls from the bombing suspects were traced to a telephone number associated with Mr Bahaji.
The German interior minister Wolfgang Schäuble said yesterday: "There have been some contacts but we don't know for sure how concrete they have been. It would have been surprising if there wasn't any contacts to Germany considering the international network of terrorism."
Mr Bahaji's name gained prominence after Mounir el-Motassadeq, a 28-year-old Moroccan, was jailed for 15 years after pleading guilty to terrorism charges relating to 9/11. A court in Hamburg was told how he had provided logistical support to an al-Qa'ida cell which allegedly included Mr Bahaji.
In Britain police are continuing to search for evidence of a plot by suspected suicide bombers to blow up airliners with liquid explosive smuggled through security checks.
Forensic specialists were searching woodland at King's Wood, near High Wycombe, Buckinghamshire, close to the scene of one of last Thursday's raids. There were unconfirmed reports that officers had found a rifle and a handgun during searches at two addresses linked to the inquiry. Counter-terrorist sources have also said that bomb-making components had been recovered.
Metropolitan Police Commissioner Sir Ian Blair stressed the seriousness of the threat still facing the country. Referring to the alleged airliner bomb plot, he said: "We have been behind this group of people for some time. What we always have to do is balance waiting to gather more evidence and make sure you get all the people, against the risk to the public by not moving in earlier.
"That's the decision that was reached last Wednesday evening. There's a point where the information reaches a level of concern that means if you don't take action it is indefensible."
He added that many other alleged plots were being investigated. "What is so concerning is that those operations can turn from being what we think is preparatory to what is clearly active in a very short time."
In Pakistan up to 17 suspects are believed to be in custody, including a Briton, Rashid Rauf, who has been officially described as a "key" suspect and is the brother of Tayib Rauf, 22, one of those arrested in Birmingham last week. The Home Office refused to confirm or deny whether the Government had formally requested his extradition. There is no extradition treaty between Britain and Pakistan, but an ad-hoc request can be made under international convention.
Sources have also suggested that the authorities in Pakistan may be looking at a connection with Matiur Rehman, a senior figure in the Pakistani terrorist group Lashkar-e-Jhangvi.
But British Security sources have dismissed claims by Pakistani officials that the trans-Atlantic bombing plot originated in Afghanistan. They said the Pakistani allegations appeared to be an attempt by Pakistan to retaliate against repeated charges made publicly by the Afghan President, Hamid Karzai, and privately by British and US diplomats and military commanders, that Pakistan was harbouring Taliban fighters carrying out attacks in Afghanistan.
Security sources said close links between Islamist extremists and the Pakistani secret police had helped in providing information. But the same links, said a senior official, meant that some of the statements from Pakistani officials "had to be treated with circumspection".
British and Pakistani investigators, however, are investigating whether money raised by appeals following the earthquakes in Pakistan last year were used to fund the alleged plot.
A charity, Jumaat ud Dawa, which is active in mosques in the UK, raised large sums in London, Manchester and Birmingham. The group is the charitable arm of a Kashmiri terrorist group, Lashkar-e-Taiba, which has been banned by the Pakistani government after pressure from the US government.
Security sources said several transactions had taken place between Pakistan and the bomb suspects, but refused to specify where they originated.
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