Police and the security services have foiled a terrorist plot to launch a poison gas attack in Britain.

A plan to use a highly toxic chemical called osmium tetroxide was uncovered after communications between terror suspects were monitored.

The chemical compound, which can be bought on the Internet for £17 a gram, attacks the eyes, turns the skin black and causes victims to choke to death in agony.

Anti-terror chiefs fear the target for an attack could have been the London Underground system, Gatwick Airport or a crowded public area such as a shopping centre.

Experts say the chemical could have been "piggy-backed" on to a conventional bomb which would disperse the invisible fumes into a crowd of people.

As they died, victims would have suffered asthma-like symptoms known as "dryland drowning".

Suspects' conversations were eavesdropped at the GCHQ electronic listening centre and police moved to disrupt the alleged plot at an early stage before any osmium tetroxide was obtained.

While al Qaida has included plans for chemical attacks in training manuals, it has so far used conventional devices and the use of poison gas would have marked a departure in its strategy for causing chaos.

Andy Oppenheimer, a nuclear, biological and chemical weapons expert for Jane's Information Group, said osmium tetroxide was an unusual choice as a chemical weapon but it could kill.

He said if terrorists were going to use it they would be likely to do so in a small bomb in a confined space.

"You can't get hold of barrels of this stuff but if you were going to set off a small device and frighten people you wouldn't really require that," he said.

"You don't think of it being used in a bomb, but you would get dispersal of the chemical in a bomb and in an enclosed space it would harm people. If people breathed in enough gas they could die."

Mr Oppenheimer said the main impact of such a bomb would be to spread panic.

l A 17-year-old arrested during anti-terror raids across the South-East was charged yesterday with conspiracy to cause explosions. Extensions of the custody period have been granted for a further eight men held under the Terrorism Act