TERRORISTS linked to al-Qaeda were planning to detonate a huge “dirty bomb” laced with a poisonous chemical in a busy shopping centre, British terrorist investigators believe.

The plot, involving highly toxic osmium tetroxide, was thwarted in its early stages after surveillance by police and security services, sources said.

The chemical is difficult to obtain in large quantities but can cause blindness, turns skin black and damages the lungs in prolonged exposure. But the bombers may have wanted to use a smaller amount as an explosive catalyst.

Combined with ammonium nitrate, it could have created a huge explosion — a terrorist “spectacular” that would have been a propaganda coup.

“Al-Qaeda are definitely interested in chemical warfare — it has long been part of their repertoire,” said Magnus Ranstorp, a terrorism expert at St Andrews University. “It is a psychological thing as much as anything. These are the type of weapons we all fear.”

The plan was uncovered during months of surveillance by MI5 and the Scotland Yard Anti-Terrorist Branch working with electronic eavesdroppers at GCHQ and the US National Security Agency.

E-mails and calls revealed discussions about osmium tetroxide, which can be bought on the internet. Ostet, as it is known in the chemical industry, sells for around £100 per gramme and is used by research scientists. All sales are carefully monitored. It is toxic if discharged in a confined space.

Investigators believe that it could have been used in attacks on targets within the M25, such as shopping centres, railway stations and the London Underground. US sources say the plot was organised by a network linked to an al-Qaeda suspect hidden in Pakistan.

Searches across the South East have yet to reveal any sign of the chemical, and investigators believe that the plot was at an early stage. Details were revealed yesterday by an American television network after MI5 had briefed the FBI.

Dave Siegrist, a bioterrorism expert at the Potomac Institute for Policy Studies, described osmium tetroxide as “a nasty piece of work”. “It leads to an asthma- like death, what we call dry-land drowning,” he said.

Islamist terrorists have previously tried to obtain osmium-based materials, believing they could be used to assemble a crude nuclear device. Chechen agents bought the isotope osmium-187, allegedly from Kazakhstan, in 2002 and attempted to smuggle them to terrorist camps.

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