The poison traces were found as six Algerians, suspected of links with al-Qaeda, were arrested following a tip off to the Security Service. One of the arrested suspects is said to be a trained chemist or a science teacher. It is believed that three other members of the gang are still at large.

Ricin, which was used to kill the Bulgarian dissident Georgi Markov in 1978, can be injected directly into the body — the most potent method — used to poison food or drink or as an aerosol.

Security sources say that this is the most dangerous discovery in Britain since the attacks of September 11. MI5 and senior detectives fear that a cache of ricin produced in the flat is in the hands of a terrorist cell. They believe that a chemical attack in London is still possible.

Documents found by The Times at an al-Qaeda safe house in Kabul in November 2001 proved that the group was experimenting with ricin.

Police have discovered that at least two of the arrested men had recently visited France and another has been in an eastern European city. Four are known to be Algerian. The others are said to be of “North African origin”.

Most of the terrorist suspects arrested in Britain and Europe since September 11 have been Algerian and it is understood that more than 20 Algerians are among the 624 detainees being held in Guantanamo Bay in Cuba.

Every doctor and hospital in the country was put on alert last night for any patient showing the symptoms of a ricin attack. There is no cure.

Tony Blair, in a bleak warning at a conference for British diplomats, said that the arrests showed “this danger is present and real and with us now and its potential is huge”.

The Prime Minister was alerted on Sunday morning as teams of detectives swooped on five addresses in North and East London. He ordered the implementation of a long-standing emergency plan to warn all medical personnel.

Last September guidance on the symptoms of ricin was placed on a website set up by the Public Health Laboratory Service on terrorist attacks, which suggests security chiefs were anticipating a threat.

Police officers donned protective clothing as they began to search the top-floor flat above a chemist’s shop in Wood Green High Road.

They found the remnants of a small homemade laboratory and rushed flasks and beakers to scientists at the Porton Down defence laboratory in Wiltshire. Police also found a supply of castor oil seeds, from which ricin is made.

It was decided not to publicise the find until the scientists were sure what the substance was. Yesterday morning Scotland Yard and Downing Street were told that small traces of ricin had been discovered in chemical equipment. One senior officer said: “You don’t need to be a highly-skilled scientist to manufacture ricin.” Chemists said last night that ricin is 6,000 times more deadly than cyanide. One gram could kill 36,000 people. Police do not believe that it would be used in a mass attack but is suitable for targeted assassinations or killing small groups of people.

David Veness, Scotland Yard assistant commissioner in charge of counterterrorism, said there should be no panic. “The message is ‘alert not alarm’,” he said. “London, and indeed the rest of the UK, continues to face a range of terrorist threats from a number of different groups.”

Police are still checking on the identities of the six men, aged between the teens and thirties, who are believed to have been travelling on false documents.

They were arrested in raids on five addresses after an investigation which began at the new year. A woman who was arrested was later released. Only one man was living at the Wood Green flat. Neighbours said that the men would go to the local off-licence but never bought alcohol.

Dr Pat Troop, deputy chief medical officer, said “This was a very low-tech, small production. If we find any more it’s likely to be a small amount.”

The symptoms of ricin poisoning are not immediately apparent. The first signs are fever, gastro-intestinal upset and coughing. Victims also suffer diarrhoea and vomiting as the poison attacks the respiratory system, kidneys, liver and immune system. Death takes between 24 and 36 hours after the victim falls into a coma.

There is no antidote but doctors can cushion patients from distress and the death-rate can be reduced by rehydration.

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