A fresh horror emerged from the ruins of post-Taliban Kabul yesterday with the discovery of Al Qaeda plans for manufacturing the biological poison ricin.

Instructions for preparing the poison were found by The Times amid dummy bombs and propaganda documents littering the cellar of an abandoned house once used as a terrorist training center.

Ricin is one of the most toxic biological agents and was notoriously used by the Bulgarian secret police when they killed Georgi Markov by stabbing him with a poison umbrella as he crossed Waterloo Bridge in 1978.

The documents uncovered in the Al Qaeda safe house reinforce the point, saying: "A dose equal to seven seeds will kill a child." The papers also spell out the agonies that a victim will go through before death, which follows within a fortnight of administration.

The Pentagon is already investigating the newspaper’s discovery of Al Qaeda’s plans for nuclear devices. Tom Ridge, America’s Director of Homeland Security, said that while they did not prove that bin Laden had managed to build nuclear devices, the documents were "certainly consistent with his statements that he would like to acquire that capacity." Donald Rumsfeld, the Defense Secretary, said of the Times report: "We saw it, we are checking it."

General Tommy Franks, the commander of the US campaign in Afghanistan, said that US Intelligence had drawn up a list of potential Al Qaeda weapons sites. "Now we are about the business of checking those sites out, as they fall under our control."

The search was given added urgency after Mullah Muhammad Omar, the Taliban leader, issued a chilling threat of reprisal against American targets. He told the BBC World Service from Kandahar: "You, the BBC, and American public radios have created a sense of concern but the current situation of Afghanistan is related to a big cause — that is the destruction of America.

"The plan is going ahead and, God willing, it is being implemented, but it is a huge task that is beyond the comprehension of human beings. If God's help is with us this will happen within a short period of time.

"Keep in mind this prediction. The real matter is the extinction of America."

The gated Al Qaeda house in Saraq Panza, Kabul, had been looted after the Taliban fled, and the two Arab doctors who worked there were beaten and killed as they tried to escape on Monday night.

The Times found ampules, syringes and pills scattered by looters across the ground floor, but the instructions for making ricin were concealed amid a mass of scattered documents listing formulae for manufacturing explosives, fuses and detonation circuits.

The instructions make chilling reading. "A certain amount, equal to a strong dose, will be able to kill an adult, and a dose equal to seven seeds will kill a child," one page reads.

Another page says: "Gloves and face mask are essential for the preparation of ricin. Period of death varies from 3-5 days minimum, 4-14 days maximum."

The instructions listed the symptoms of ricin as vomiting, stomach cramps, extreme thirst, bloody diarrhea, throat irritation, respiratory collapse and death.

Originally manufactured in Russia, ricin was used by British scientists to develop a "W bomb" during the Second World War, but it was never used. It was also found by U.N. weapons inspectors in Iraq in the 1990s.