The manual, posted on October 6 on a forum titled Al-Firdaws, or Paradise, contains 80 pages of instructions and pictures of kitchen bomb-making techniques. It is divided into nine lessons under the overall heading The Nuclear Bomb of Jihad and the Way to Enrich Uranium, and is dedicated as a “gift to the commander of the jihad fighters, Sheikh Osama Bin Laden, for the purpose of jihad for the sake of Allah”.

As well as describing how to make a nuclear bomb from enriched uranium — impossible for the layman — the manual explains how to make simple bombs that can blow up anything from electrical generators to petrol stations.

The site encourages its readers to look for materials such as radium, which it says is an “effective alternative to uranium and available on the market”. It is unclear who the author is or where he is based: he describes himself simply as “Layth al-Islam”, or the “Lion of Islam”, belonging to a group called “the Black Flags”.

“Fight them so that Allah will punish them at your hands and will put them to shame and will give you victory over them,” he writes, quoting the Koran. “Perhaps nuclear weapons represent a technology of the 1940s. However, the Crusaders, the allies of the Satan, Allah’s curse be upon them, insist on depriving the jihad fighters of the right to have these weapons.”

The site’s appeal is evident from the enthusiasm of its correspondents. One of the most recent, Mariyam al-Jihadiyya, writes: “God bless you for this precious topic . . . fight them, through your hands God tortures them . . . and heal the hearts of the faithful people.” Beneath she includes a couple of pictures for her hero. “I love you, Osama,” she writes.

Other users complain that not all the site’s links are activated, and several urge caution. “Don’t talk about things you don’t understand,” writes one. For enthusiasts there are links to a mailing service that provides regular updates on bomb-making techniques.

Nuclear physicists were alarmed by the site. “Normally you just get generic principles, but this appears to be more like a proper instruction manual,” said John Hassard, reader in physics at Imperial College, London. “The thing about this website that is striking is that it is very particular. A lot of effort has been put into it.”

He said that while it was highly unlikely that amateur bomb-builders could get hold of fissile material, smuggling networks with access to nuclear materials from the break-up of the Soviet Union could use the information.

“It is a very real threat and one which we can’t afford to ignore,” he said. “I would say this is public enemy No 1.”

Experts on Al-Qaeda said the organisation appeared to be moving from a phase where it preached a fatwa permitting the use of weapons of mass destruction — issued two years ago — to one where it encourages its followers to produce both “dirty” bombs and smaller devices similar to those used in the London Tube attacks.

“Al-Qaeda strives to move directly from the stage of obtaining the WMD to the stage of using it,” said Matti Steinberg, an Israeli expert on the organisation. He said efforts by Al-Qaeda, whose members are Sunni Muslims, to produce a nuclear weapon also reflected its fear that Shi’ite Iran was on the brink of producing a bomb. Bin Laden wanted to “balance the efforts by Iran to obtain the first Shi’ite bomb by building the first Sunni one”.

While assessing the website’s influence on young British Muslims is difficult, terror experts believe it is an important potential recruiting tool.

Jeevan Deol, a terrorism analyst at the London School of Oriental and African Studies, said that while Al-Qaeda could not match western military capabilities and intelligence, its use of “cyberwarfare” helped redress the balance.

“They are using the web in a focused way for propaganda and recruiting,” said Deol. “Some jihadi kid in Leeds clicks on it and thinks, ‘Wow, 50,000 hits — we don’t see Osama on telly any longer but we’re big, we’re bad and extremely engaged in all these things’.”

Additional reporting: Widiane Moussa and Flora Bagenal

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