Doctors have been warned to look out for signs of exposure to the potentially lethal poison ricin, after it was found by anti-terrorist police at an address in north London.

Six Algerian men are being questioned in connection with the discovery, made following an intelligence tip-off.

The men were arrested on Sunday morning and are in their late teens, 20s and 30s.

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

The intelligence services are said to be "shocked and worried" by the discovery and are looking at possible links with suspected Islamic extremists.

Forensic analysis

The arrests involved officers from the Anti-Terrorist Branch, Special Branch and the Security Service.

Ricin poison Tiny amount can kill No known antidote Causes gastroenteritis, vomiting and seizures

More details on ricin poisoning

Castor oil beans - from which ricin is made - and equipment and containers for crushing the beans were found at a flat in Wood Green, north London, where one of the men was arrested.

Police said forensic analysis of the address - where a small quantity of material tested positive as ricin - was continuing, although they do believe the poison was made there.

It was identified by scientists at the Defence Science and Technology Laboratories at Porton Down in Wiltshire.

Police have not ruled out the possibility that some ricin may already have been distributed, although they believe it is highly unlikely.

They will also be looking at whether the group were part of a wider operation, possibly involving the manufacture of other chemicals.

'Real evidence'

BBC home affairs correspondent Margaret Gilmore said: "For six months now MI5 and the anti-terrorist branch have been getting intelligence reports indicating that extreme groups want to launch a chemical, biological or radiological attack.

"Now we're being told this is probably the first real evidence they were trying to do this here in the UK."

It is thought that whoever made the poison did not have the capability to make a bomb, but they could have aimed to create panic by trying to kill small numbers of people.

Defence minister Geoff Hoon described the discovery of ricin as a "disturbing development".

Ricin is considered a potential biowarfare or bioterrorist agent and is on the Centre for Disease Control and Prevention's "B" list of agents - considered a moderate threat.

The flat in Wood Green was raided on Sunday

It is relatively easy to manufacture in small amounts but would be considered an unusual agent to use for a mass attack as it must be ingested or injected to take effect.

It was also the toxin thought to have been used to murder dissident Bulgarian Georgi Markov, who was stabbed on Waterloo Bridge in London with a poisoned umbrella in 1978.

Mr Blair's official spokesman stressed there had been no specific intelligence about how the ricin was to have been used.

It could have been produced by amateurs in very small quantities Sir Timothy Garden, former assistant chief of defence staff

Deputy Chief Medical Officer Dr Pat Troop said: "While our message is still 'alert not alarm', we would re-iterate our earlier appeals for the public to remain vigilant and aware and report anything suspicious to police."

A Department of Health spokeswoman said all GPs and doctors had been told to look out for possible cases of ricin exposure.

Sir Timothy Garden, former assistant chief of defence staff, told BBC News 24: "If it's a significant quantity then it's a worry because this is a poisonous agent which would require a lot of work to produce in a major quantity for use by terrorists."