Terror gas attack on Tube foiled by security agencies

A planned poison gas attack, with the London Underground as a likely target, has been foiled by the security agencies, it was claimed yesterday.

A planned poison gas attack, with the London Underground as a likely target, has been foiled by the security agencies, it was claimed yesterday.

The plot allegedly involved detonating a combined chemical and explosive "dirty bomb", producing fumes that can choke victims in a confined place. The conspiracy was uncovered after British and United States intelligence intercepted telephone calls within this country, as well as calls to Pakistan.

As well as Tube trains, passenger terminals at London Gatwick and Heathrow airports are believed to have been in the list of targets. However, it is believed the plot was exposed before the alleged terror cells had reached the position to carry out an attack. Security sources said yesterday that the attack involved the use of a chemical called osmium tetroxide in an explosion. However, security experts last night questioned whether such a chemical could be used in an attack by terrorists.

The substance turns from solid to gas in confined space and is highly corrosive to eyes, skin tissue and lungs, producing a symptom called "dry land drowning". It is believed that a fertiliser-based explosive would have been part of the package.

The telephone calls between the alleged plotters were intercepted by GCHQ in Cheltenham, Gloucestershire, and the US National Security Agency. Al-Qa'ida leaders have been warning of a chemical attack since the 1999 bombings of the World Trade Centre in New York, and handbooks have been produced on how to manufacture "dirty bombs".

However, so far, the organisation and its affiliates had only carried out attacks with conventional explosives, and some experts pointed out yesterday that osmium tetroxide, which has a legitimate scientific use, did not fit the usual profile of a typical chemical warfare agent. They said handling the chemical would be potentially dangerous for would-be bombmakers.

Professor Alastair Hay, professor of environmental toxicology at Leeds University, said the substance, known as a "rare catalyst" a chemical that speeds experiments, could be used to speed up an explosion.

But he added: "It would not be in the same category as some radioactive substance which would continue to emit radiation and cause a problem in terms of clean-up. This would be something present, like a heavy metal lead, in the environment. I don't think it would be a major hazard and clean-up would not be a major problem."

Dr Steve Simpson, a senior lecturer in chemistry at the University of Salford, who works with the substance, said: "If you get the vapour in your eyes, even a small amount, it can turn them brown or black and you could be permanently blinded. It is so volatile that one could be in appreciable danger just opening a bottle. If one had 10 grams and opened it up in a normal-sized room, within a couple of minutes it would cause people's eyes to stream. Some people might feel a bit tight in the chest. It would cause massive panic."