Britain was on a poison alert last night after police discovered traces of the lethal biological toxin, ricin, at a London address.
The find came in the early hours of Sunday as anti-terrorist officers, acting on intelligence, raided houses in north and east London and arrested six men and a woman.
The six men, all said to be of North African origin, were still being questioned last night as part of an intensive police investigation to establish how and when they entered the UK and whether any quantities of the toxin remain undetected. The woman was earlier released.
Anti-terrorist, Special Branch and MI5 officers believe their joint operation foiled a planned terrorist attack. The British Prime Minister, Mr Tony Blair, said the discovery and arrests highlighted the continuing threat posed by international terrorism and showed "this danger is present and real and with us now."
While the political and security authorities last night sought to manage and contain public alarm, stressing that only small traces of the toxin had been found, the Defence Secretary, Mr Geoff Hoon, admitted that the discovery was "a disturbing development".
In a joint statement the Assistant Commissioner of the Metropolitan Police, Mr David Veness, and the Deputy Chief Medical Officer, Dr Pat Troop, confirmed that 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 was sent for analysis at the Defence Science and Technology Laboratories at Porton Down, and a small amount of the material recovered tested positive for the presence of ricin.
Ricin is a "contact" poison and can be fatal if inhaled or ingested in even tiny amounts. A by-product of making castor oil, it is considered a potential bio-warfare or bio-terrorist agent and is on the Centre for Disease Control and Prevention's B list of agents, considered a moderate threat.
Experts last night said there was no evidence that any attempt had ever been made to incorporate ricin in a weapon of mass destruction. It is believed the toxin was used in the murder of a Bulgarian dissident, Mr Georgi Markov, who was stabbed with a poisoned umbrella on Waterloo Bridge in London in 1978.
Large quantities of the toxin were also reportedly found by British and American forces in caves in Afghanistan last year.
Mr Blair's spokesman said there was no specific intelligence as to how ricin might have been used in any planned terrorist attack, and Dr Troop said it appeared any production had been on a very small scale.
The police were working closely with the Department of Health, which alerted health authorities across the country to the developments. Steps were being taken to ensure the health service was able to provide advice to members of the public, and Mr Veness and Dr Troop said if any new developments had implications for public safety the public would be informed immediately.