Police said Tuesday they found traces of ricin — a deadly poison twice as potent as cobra venom — in a north London apartment and arrested six men of North African origin in connection with the virulent toxin that has been linked to Al Qaeda terrorists and Iraq.

London police said material seized at a flat in the Wood Green neighborhood on Sunday had tested positive Tuesday for traces of the toxin, tiny amounts of which can kill an adult. There is no antidote.

Prime Minister Tony Blair, speaking to a meeting of British ambassadors, said the find highlights the threat posed by weapons of mass destruction.

"As the arrests...show, this danger is present and real, and with us now, and its potential is huge," he said.

Ricin (pronounced RICE-in) is derived from the castor bean plant, which is grown around the world. The poison is relatively easy to produce, and Andy Oppenheimer, a chemical and biological weapons expert at Jane's Terrorism and Security Monitor, said its presence in London did not necessarily indicate a connection to any outside group or country.

In Washington, U.S. officials said no Al Qaeda links had yet been established to the London arrests, but that it was a matter being investigated.

Anti-terrorist police said they arrested the six men of north African origin under the Terrorism Act during raids in east and north London and seized "a quantity of material and items of equipment" at the Wood Green apartment.

Police did not identify the men and refused to specify what country or countries they were from, saying only that they were in their late teens, 20s and 30s. They were not immediately charged with a crime.

The British Broadcasting Corp. reported on television Tuesday night that the men were Algerian, but Scotland Yard refused to confirm that.

A woman arrested in the raids was released, authorities said.

Police said the arrests were prompted by "receipt of intelligence" but gave no other details.

"We have previously said that London — and indeed the rest of the U.K. — continues to face a range of terrorist threats from a number of different groups," police anti-terrorist branch chief David Veness and Deputy Chief Medical Officer Dr. Pat Troop said in a statement.

Blair's spokesman said he knew of no specific intelligence about how the suspects may have planned to use the ricin.

The Department of Health said doctors around Britain had been informed of the find and warned to look for symptoms of ricin exposure, including fever, stomach pains, diarrhea and vomiting.

Ricin causes diarrhea so severe that victims can die of shock from massive fluid and electrolyte loss.

Oppenheimer, the weapons expert, said injecting ricin was an effective way of targeting individuals as was the case of Bulgarian defector Georgi Markov, killed in London in 1978. Police said ricin was in a pinhead-sized pellet injected into Markov's thigh, but couldn't confirm the widely reported theory that he was jabbed by a rigged umbrella.

Oppenheimer said terrorists could kill large numbers of people with ricin if they put it into aerosol, a job he described as tricky but not impossible. A crowded, enclosed environment like the London subway would probably be the most appealing target, he added.

"It's just one of these horror scenarios which people are very frightened of at the moment," he said. "You only need milligrams to kill somebody."

For decades, Londoners lived with Irish Republican Army bomb attacks, but the specter of biological terrorism is frighteningly new.

In November, the government issued — and then hurriedly withdrew — a statement warning that Al Qaeda might be prepared to use a radiological device or poison gas in Britain. The warning was replaced with a more general alert of terrorist threats.

On a tape released in November, a speaker believed to be Al Qaeda chief Usama bin Laden castigated U.S. allies that have joined the war against terrorism, specifically mentioning Britain, France, Italy, Canada, Germany and Australia.

U.S. officials said in August that the Islamic extremist group Ansar al-Islam had tested ricin along with other chemical and biological agents in northern Iraq, territory controlled by Kurds, not Iraqi President Saddam Hussein. Ansar is allegedly linked to Al Qaeda.

United Nations weapons inspectors who left Iraq in 1998 listed ricin among the poisons they believed Saddam produced. U.S. troops also found traces of the substance at suspected Al Qaeda biological weapons sites in Afghanistan.

Amateur American scientists with no links to terror groups have also produced the poison at home.

In Janesville, Wis., Thomas Leahy pleaded guilty to possessing ricin in 1998. The FBI arrested Kenneth Olsen in Spokane, Wash., last summer for allegedly manufacturing it, a charge he denies.