British police have foiled a poison gas attack on London, it was claimed today.
A highly toxic chemical called osmium tetroxide was to be used in a device, ABC News reported in the United States. The BBC also said it had learned of the plot.
The chemical compound, which can be bought on the Internet, causes victims to choke to death in agony. It also attacks the cornea and can lead to blindness.
It could have been used by terrorists to target the Tube in London, Gatwick Airport or a busy shopping centre, ABC News said.
Police and security services in Britain were alerted after mention of the chemical was picked up at the GCHQ electronic listening centre, ABC News said. They moved to stop any possibility of it being used.
Osmium tetroxide has a pungent smell and comes from the Greek word for "stench". It is used in research laboratories as a staining agent in electron microscopy. Despite being very dangerous, small amounts can be obtained through the Internet.
Experts say it could be "piggy-backed" on to a conventional bomb which would see the invisible chemical dispersed into a crowd of people. As they die, victims would suffer asthma-like symptoms known as "dryland drowning".
Police in Britain have not said that they have found any osmium tetroxide.