Six men were being questioned by detectives in London today after traces of the deadly poison ricin were discovered, Scotland Yard said.
The six men of north African origin were arrested after an operation by the anti-terrorist branch in north and east London.
The yard said "equipment and materials" were found at an address in Wood Green where one of the men was also arrested. A woman who was also arrested has been released.
Ricin was the poison used to murder Bulgarian exile Georgi Markov in a political assassination in London in 1978.
Today's news comes weeks after security sources warned of a possible terror attack involving London underground.
In a joint statement issued this afternoon, anti-terror chief Mr David Veness and Deputy Chief Medical Officer Dr Pat Troop said: "In the early hours of Sunday, January 5th, 2003, six men and one woman, were arrested under the Terrorism Act 2000 at premises in north and east London by officers from the Metropolitan Police anti-terrorist branch.
"The six men, aged in their late teens, 20s and 30s, remain in custody and are being interviewed by anti terrorist branch officers. The woman has been released.
"This successful joint operation between the anti-terrorist branch, MPS Special Branch and the Security Service follows receipt of intelligence.
"A quantity of material and items of equipment were found at a residential premises in Wood Green, north London, where one of the men was arrested.
"This material has been analysed at the Defence Science and Technology Laboratories at Porton Down.
"A small amount of the material recovered from the Wood Green premises has tested positive for the presence of ricin poison."
The statement continued: "Ricin is a toxic material which if ingested or inhaled can be fatal."
Clinical features of ricin poisoning include fever, upset stomach, and coughing. Breathing in ricin causes serious lung damage and breathing difficulties. Swallowing the poison causes irritation of the gut, gastroenteritis, bloody diarrhoea and vomiting.
Effects on the central nervous system have been reported including seizures. Effects of exposure to ricin may be delayed for some hours and patients who develop a fever may consult their own doctors.
A warning about the effects of the poison has been placed on the British Public Health Laboratory Service website.
PA