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 new 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 in an enclosed space they could die."

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

"It would be used to up the ante, to frighten people. It's just the very idea that anything could be used even if it was a common chemical.

"There really is a growing threat of terrorists getting hold of this sort of material.

"There are so many chemicals that could be used."

Conservative homeland security spokesman Patrick Mercer said: "There is no doubt that terrorists are very keen on looking at biological and chemical weapons."

Liberal Democrat leader Charles Kennedy said reports of the plot were "very troubling".

"It is reassuring at the same time, obviously, that apparently the security and intelligence services have been able to foil something," he said.

"Thank goodness success appears to have been achieved."

But the security services faced an "almost impossible conundrum", added Mr Kennedy. "You want to be able to inform people for reassurance purposes that the closest possible surveillance and infiltration does take place.

"You don't, of course, want to compromise your sources and at the same time you don't want to engender unnecessary alarm which cuts across our civil liberties."

Alastair Hay, professor of environmental toxicology at Leeds University, said there was a need for the Government to tell people what the risks of chemicals are.

He said: "There is a real need for the Government to engage with people and say that this is what could happen, what the response of the authorities would be and what should people do.

"The Government is probably nervous about worrying people but, equally, people get worried if they don't know what is going on."

l A 17-year-old arrested during a series of anti-terror raids across south-east England 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.