At least 11 of the hijackers in the American atrocities passed through Britain and may also have stayed here, according to information given to Scotland Yard by the FBI.
The Metropolitan Police is also investigating whether the other eight terrorists responsible for the suicide attacks and members of their back-up teams were in Britain in the month before 11 September.
The FBI has given anti- terrorist officers the names of the 11 hijackers who arrived in America from British airports. Britain has long been suspected of being used for fund- raising and recruitment by extremist Muslim groups but, if evidence emerges that terrorists have been using it as their main base, that will raise fears about attacks there.
The FBI has also asked British authorities to trace more than 100 other people, some of whom are suspected of being involved in planning and financing the assaults.
Inquiries have been hampered by the hijackers' use of multiple names and stolen identities. Additional problems have been caused by loopholes discovered in British immigration procedures and airport computer systems that may have allowed the terrorists to embark at Heathrow or Gatwick without the knowledge of the authorities.
The anti-terrorist branch has so far arrested four suspects on behalf of the FBI, two of whom have been released. A district judge yesterday gave the police more time to question Lotfi Raissi, an Algerian pilot, 27, who was arrested on Friday in Berkshire, and a student in his 40s. They can be detained until tomorrow.
The police are also still trying to locate the former girlfriend of Habib Zacarias Moussaoui, a former resident of Brixton, south London, who is though to be the "20th member" of the suicide squads. He is currently being held in America where he was arrested shortly after the attacks.Reuse content