Intelligence agents in the UK and US have foiled an alleged chemical bomb plot in Britain, the BBC has learnt.
Osmium tetroxide is not known as a chemical weapon agent
The plot was believed to involve detonating a combination of explosive and a chemical called osmium tetroxide.
Experts say in gas form it could be lethal in a confined space.
The plotters were thought to be sympathetic to the aims of al-Qaeda and the intended target was believed to be British civilians, probably in London.
The chemical has a legitimate scientific use for research but is highly destructive to peoples' eyes, lungs and skin.
The plot was foiled after US and British intelligence intercepted communications between the plotters and it is not thought that they had managed to obtain any of the chemical, osmium tetroxide.
The target was thought to be areas in which there would be concentrations of people, possibly within a confined space.
The UK has been on a high state of alert since bombings in Madrid on 11 March claimed 191 lives.
Metropolitan Police commissioner Sir John Stevens said a terror attack on London was inevitable but Home Secretary David Blunkett has tried to play this down.
Alastair Hay, Professor of environmental toxicology at Leeds University, said osmium tetroxide was a rare catalyst - a chemical that speeds experiments - and could potentially make an explosion occur more rapidly.
Laboratory chemical which requires precautions when handled
Used in scientific experiments, not known as a chemical weapon agent
Toxic and irritant, even in small amounts
Direct contact causes skin and eye damage
But Professor Hay told the BBC it would have to be obtained from a specialist chemical supplier and it did not fit the profile of a typical chemical warfare or dirty bomb agent.
"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 like lead, in the environment. I don't think it would be a major hazard and clean up would not be a major problem," he said.
Security expert Dr Sally Lievesly said terrorists were well aware of the psychological impact a chemical bomb could create.
She told BBC One's Ten O'Clock News: "The emergency services would be faced with a terrible scene. They would have to kit out, they'd be delayed and the injuries they'd be seeing would be very bad. So their job is a very difficult one.
"The public, with these types of attack, if the public are not prepared this then becomes a weapon of psychological terror."
On Tuesday night the intelligence services, including MI6, were declining to comment on what appears to be a successful operation by them and the police.
Earlier, a Whitehall official told the BBC that, had this plot succeeded, it would have been the most serious attack on Britain in many years.