Suspected terrorists in London have reportedly tried to buy half a tonne of the toxic chemical saponin with the aim of launching a deadly poison attack.
Ricin is found naturally in castor beans
Their plot came to light when the supplier, suspicious about the size of the order, contacted the police, according to the Financial Times.
Saponin makes biological cells more susceptible to other toxic chemicals.
It could have been mixed with ricin and smeared on surfaces in public places, experts told the newspaper.
Less than a milligram of ricin, found naturally in castor beans, can kill an adult.
But one expert cast doubt on whether the alleged plot would have worked.
Jonathan Tucker, from the Center for Nonproliferation Studies at the Monterey Institute, Washington, told the paper he doubted it would be possible to absorb the large protein molecules of ricin through the skin.
A company giving a London post office address reportedly tried to buy 500 kg of saponin from Amersham Biosciences in autumn 2002.
Business development director Lennart Arlinger told the newspaper representatives had described its intended use as "a fire retardant
on rice intended for human consumption".
A Metropolitan Police spokesman told BBC News Online they were "not prepared to discuss the matter".
He said he was not aware of any charges being brought in relation to the alleged incident.
Home Secretary David Blunkett also refused to confirm the report.
But he told BBC Radio 4's Today programme that planned terror attacks on London had been foiled.
Counter terrorist agencies were foiling similar operations "all the time", he said, adding he was "sick and tired" of people pretending there was not a terrorist threat.