London: As Britain's security services tried on January 8 to establish if terrorists have obtained the deadly substance ricin, traces of which were found during a raid at a North London flat, national health services were put on alert to meet any exigency.

British police was also trying to work out how much of the substance, one of the world's most powerful poisons and twice as deadly as cobra venom, was made at the flat they raided on January 5.

Six Algerians, suspected of links with al-Qaida terror network, were arrested after the raid in Wood Green and are being interrogated by Scotland Yard's anti-terrorist branch. One of the arrested suspects is said to be a trained chemist.

After tests at Defence Science and Technology Laboratories, samples showed that residue found was ricin, a toxin with no known antidote and whose manufacture has been linked to supporters of Osama bin Laden, the government has alerted health services workers to the symptoms of ricin poisoning and given them details on how to treat it.

British Prime Minister Tony Blair said the arrest showed the continued threat of international terrorism was "present and real and with us now and its potential is huge".

Security forces are trying to discover the links of those behind the manufacture of ricin with al-Qaida or other terror groups, and whether other chemicals have been made.

It was the first time that such a deadly poison was found in Britain since September 11, 2001 strikes on US, following which Britain has been on high alert.

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

Castor oil beans from which ricin is made, and equipment and container for crushing the beans were found in the flat where one of the men was arrested.

Mi5, the British intelligence service and senior detectives fear that a cache of ricin produced in the flat is in the hands of a terrorist cell and that a chemical attack in London is still possible.

Detectives also believe the six men may have accomplices who are still at large.

According to the police, the suspects, who have been under investigation for a month following a tip off to Mi5, have no connection to Iraq.

Though the poison, which was known to scientists as "compound W", can be used in a bomb - a device was developed by Britain but not used in the Second World War - it is more commonly known as an effective method of discreet murder.

David Veness, Head of Specialist Operations, Scotland Yard said the six men were arrested, along with a woman, under the Terrorism Act 2000. The woman has been released later. The operation involved the anti-terrorist branch, Scotland Yard's special branch and Mi5.

"Ricin is a toxic material which if ingested or inhaled can be fatal. Our primary concern is the safety of the public and the police have worked closely with the Department of Health throughout," Veness told reporters.

"The department is now alerting the health service... We have previously said that London and indeed the rest of the UK, continues to face a range of terrorist threats from a number of different groups," he added.

PTI