Many of the web-pages clearly predate the most recent terror incidents such as London's 7 July 2005 tube bombings and many were also clearly online before New York's 9/11 attacks.

In the months after the 2001 attacks on the US, there was a reluctance among news media to highlight the availability of information online in relation to homemade bomb-making instructions.

But such is the choice that internet users have today in relation to such information that most if not all publications have at some time or other written extensively on the information available online, including the Guardian and Observer newspapers in the UK.

Most websites are run by ordnance enthusiasts. The people who run the geocities. com/pyroklutchwebpages, for instance, suggest that "making explosives on your own can be a lot of fun for the experimenting youth or older people looking for a good time".

That website gives instructions on how to manufacture napalm, Mexican M-90 and film canister bombs, all of which can be made using easily found ingredients in hardware shops.

"There are a lot of homemade mixtures you can concoct from some very common materials that are innocent in themselves, " said Andy Oppenheimer, editor of Jane's Nuclear, Biological and Chemical Defence Directory.

"We are talking about common, everyday chemicals that are used in perfumes, cosmetics, drain cleaner, batteries, or could for example be stolen from school labs. . . these materials are easy to obtain and hard to detect, and could be smuggled in small amounts in small containers because it doesn't take much to blow an aircraft up."

However, Oppenheimer said that the ingredients would have to be mixed together in the correct proportions.

And even if they are mixed properly, the mixture would be very unstable and sensitive to shock or heat.

Police believe acetone peroxide triacetone triperoxide (TATP) was the home-made explosive used on London's transport system on 7/7 last year.

Those attacks killed 56 people, including four young Muslim fundamentalist suicide bombers.

TATP is made with commonly available chemicals such as sulphuric acid, which is used to clean drains, and hydrogen peroxide, which is used in hair dyes, and acetone.

It is likely that the young bombers obtained this information online using some of the many al-Qaeda-linked Arabic language websites that are available.