A plot to unleash a deadly chemical attack on Gatwick airport has been foiled, US intelligence has claimed.

Operatives at the Government's GCHQ electronic listening centre apparently intercepted terrorist plans to set off a device loaded with highly-toxic osmium tetroxide.

The chemical compound, which can be bought freely on the internet, causes victims to choke to death - termed "dry-land drowning".

It can also cause blindness and experts say it could be "piggybacked" on to a conventional bomb to disperse the invisible substance into a crowd.

Sources within the US intelligence service told the ABC News network of the alleged plot, suggesting Gatwick, the London Underground or a busy shopping centre were among the intended targets.

Police and security agents have refused to confirm any details, although British authorities are said to have feared it could potentially have been one of the worst attacks against the UK to date.

The GCHQ intercepts suggested an alleged terrorist operation was being run from Pakistan by a suspected al Qaida figure.

A spokesman for New Scotland Yard, which is running the investigation, refused to comment on the ABC report.

He said any queries about arrests made under the Prevention of Terrorism Act were "unhelpful".

A spokesman for Gatwick airport operator BAA said: "There is a criminal investigation and we are not able to comment while that is ongoing."

The Argus contacted one manufacturer and supplier of osmium tetroxide, which can also be bought over the phone.

A spokesman said: "The chemical is used almost exclusively by universities for specialised microscopic studies.

"It is extremely unusual to receive large orders and if one was placed then alarm bells would ring."

Home Secretary David Blunkett used the unconfirmed details of the alleged plot to justify his decision to bring in tough anti-terror measures.

He said: "All of us, for two-and-a-half years, have been indicating that that is precisely what the network called al Qaida, in its loose form, is actually about.

"People should be praising and being very grateful we have the security and counter-terrorism services we do because they are doing a first-class job.

"They have got my whole-hearted backing because this is the only protection we really have."

Opposition parties said reports of the plot came as no surprise but were "very troubling".

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

He said: "The Govern-ment is probably nervous about worrying people but equally people get worried if they don't know what is going on."