The security services hope to track down terrorists in Britain by appealing to users of extremist Islamic websites for information.
MI5 officers have posted messages on websites aimed at rooting out members of terror cells who may have played a part in planning the 11 September atrocities in New York and Washington, a Home Office spokesman said.
It is hoped that the tactic will shed further light on the terrorist attacks and stop any copycat missions on the British mainland.
A Home Office spokesman said: "The purpose is clear. We are asking members of the public to telephone us if they have any information that could help us track down terrorists.
"To do this successfully we must rely on the help of the general public."
Several extremist websites have been targeted including islah.org, set up by a Saudi opposition group, which is known to be accessed by supporters of Osama bin Laden, and a Chechen site which advocates jihad, or holy war.
Written in Arabic, the MI5 message says: "The atrocities that took place in the USA on 11 September led to the deaths of about 5,000 people, including a large number of Muslims and peoples of other faiths.
"MI5 (the British Security Service) is responsible for countering terrorism to protect all UK citizens of whatever faith or ethnic group. If you think you can help us to prevent further outrages, call us in confidence on 020 7930 9000."
MI5 reportedly had 671 replies within the first 10 hours, 16 of which attacked the allied coalition's aims in Afghanistan.
The MI5 switchboard is staffed by intelligence officers who are seeking to widen the service's web of informants.Reuse content