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“It was the first tangible sign that was something more than fundraising,” one senior officer said.

The Porton Down research establishment tested the instructions and said that they were viable for making lethal doses of poison.

Khalef was working in a food factory in East Anglia, but there was no evidence in Thetford or Ilford of any attempt to make toxins. Detectives feared that they were in a race to find the poisoners before they could strike. Intelligence agencies and police forces across Europe and northern Africa were on the alert but the trail was cold.

One source said: “Imagine the impact of discovering that we had found people who were apparently interested in mounting attacks in the UK using unconventional weapons.”

An emergency summit was held in London to brief 200 senior figures from Whitehall, police forces and the transport industry about the threat of a poison attack, possibly on the London Underground.

In November 2002, French Intelligence alerted police to the arrival in Britain of Rabah Kadre, a key figure in the Algerian Islamist terrorist network.

Fearing that Kadre’s arrival might be the catalyst for action, the authorities arrested him. “We could have let him run,” one security source said. “But he is a dangerous man and too much of a risk.”

In Paris, four of his associates were arrested in a raid in which police found a formula for cyanide and a chemical warfare protection suit. Today Kadre remains in a British prison, fighting extradition to France.

His detention provoked media stories about a possible cyanide attack on the Tube. But these were off the mark. Those chasing the gang still did not know what was planned.

That knowledge was not acquired until the end of December, when Algeria reported that it had detained a terrorist suspect called Mohammed Meguerba. He told his interrogators that he had been part of a group in London planning attacks using homemade chemical weapons. He had fled Britain two months before, after his arrest during the September police raids. He had been bailed and was ordered by his superiors to leave Britain. The 27-page memo on his interrogation, which may have involved the use of torture, detailed the plan to make poisons and gave the first hint of possible targets. It would not be a mass attack, but on chosen individual civilians.

Meguerba said that his gang had discussed smearing toxic pastes or liquids on car and door handles around Holloway in North London. The aim was to trigger widespread fear and panic. The leader of the plot was identified as a man called “Nadir”, with whom Meguerba claimed to have filled two Nivea cream pots with ricin. Those pots have never been found. But one discovered in a wardrobe contained a nicotine poison. He did not know the address where he and “Nadir” had worked on their formulas, but his description of how he travelled there led police to a two-bedroom flat above a pharmacy on Wood Green High Road in North London. Number 352b was rented by Sidali Feddag, a young asylum-seeker. On January 5, 2003, the flat was raided.

Police found the handwritten originals of the instructions for poison and instructions for making explosives and detonators. Plastic cups, in which apple pips and cherry stones were being collected, were found along with castor beans and bottles of acetone, packets of plastic gloves, thermometers, digital scales and funnels. In a holdall was £4,000 in cash.

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