We are asking members of the public to telephone us if they have any information that could help us track down terrorists

Home Office spokesman

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 it relies on the help of the general public."

The spokesman said several extremist websites had been targeted.

BBC News Online has found the message in the chatroom of a website belonging to a Saudi opposition group, the Movement for Islamic Reform in Arabia.

While not endorsing the 11 September attacks, the group is sympathetic to Bin Laden's aims.

"Bin Laden is a good warrior, faithful to the cause," it says on its website.

MI5 is also reported to have placed the message on a Chechen site which has advocated holy war.

Protection plea

Written in Arabic, the MI5 message said: "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 call us to prevent further outrages, call us in confidence on 020 7930 9000."

So far, most of the 20 replies in the chatroom of the Movement for Islamic Reform in Arabia appear to be either hostile or mock the initiative.

"I know a nice anthrax spore which was trained at al-Qaeda camp I could send it to you! Give me $5m and I will help you," said one message.

One person sent an image of a nuclear explosion as his reply.

Another said "Shame on you Blair," with a cartoon of the prime minister with breaking wind.

Some questioned whether the appeal was real.

"Is this a joke?" asked one contributor.

"Definitely a joke!" answered another.