The toxic chemical, osmium tetroxide, a substance which could be used to boost an explosion, was mentioned in conversations intercepted by GCHQ, the government's electronic eavesdropping centre.

In large doses, the substance, which can be obtained on the internet but is mainly used in chemical experiments, can be fatal.

Most experts suggested that its most likely use would have been as a booster for a bomb made with more common explosives.

If osmium tetroxide was used in this way, the result would not be a particularly "dirty bomb" since its harmful effects would be largely destroyed in the explosion, experts said.

It is possible the substance could have been "piggy-backed" on to a bomb, though chemical weapons experts said it would not be an obvious choice.

But it could have caused considerable harm and panic in a confined or crowded space, such as the London Underground or an airport.

Al-Qaida bombmakers have previously added small quantities of booster explosive, detonators and timers to ammonium nitrate to trigger huge blasts including the recent attacks on the British embassy in Istanbul and the British compounds in Saudi Arabia.

Scotland Yard refused to comment on the alleged "chemical plot" or say whether it was connected to the seizure of half a tonne of ammonium nitrate fertiliser and the arrests of nine men in a counter-terror operation last week.

However, a source familiar with the intelligence operation said yesterday: "Nothing's been found. It's talk of stuff rather than the stuff itself."

The alleged plot - intercepted by GCHQ, possibly with the help of its American equivalent, the National Security Agency - was first revealed early yesterday by America's ABC News.

It claimed the chemical could have been used by terrorists to target the tube in London, Gatwick airport or a busy shopping centre.

GCHQ's role may explain the unusual reticence yesterday of the police and security services. The product of its intercepts are treated with much more sensitivity than human surveillance operations.

Last week's arrests - a number of which were near Gatwick airport - involved all three of Britain's security and intelligence agencies, MI5, MI6 (which operates abroad) and GCHQ.

The youngest of the men arrested last week, a 17-year-old, from Crawley, West Sussex, was charged last night with conspiracy to cause explosions with intent to injure or damage property. He will appear at Bow Street magistrates court today.

Detectives are continuing to question the other eight, aged between 18 and 32, at the high security Paddington Green police station in central London and have extensions to hold them until tomorrow.

More than 700 police officers were involved in the initial eight arrests in London and south-east England last Tuesday, and a 27-year-old man was seized on Thursday. Under anti-terrorist legislation, each of the suspects can be held for 14 days from the time of their arrest.

All are British citizens and eight, including the 17-year-old, are of Pakistani descent, and one is of Algerian origin.

Momin Khawaja, a 29-year-old Canadian, appeared in court in Ottawa on Friday on related charges, and his father, Mahboob Khawaja, is believed to have been detained by the Saudi Arabian authorities in connection with the bomb plot.

http://www.guardian.co.uk/terrorism/story/0,12780,1187367,00.html