Kamel Bourgass was jailed for life in June 2004

Kamel Bourgass, 31, is already serving a life term after being convicted of murdering Detective Constable Stephen Oake during a 2003 raid in Manchester.

Four other men were cleared last week of taking part in a conspiracy. A second trial has been abandoned.

Anti-terror chief Peter Clarke said a "real and deadly threat" was averted.

The impact on the public, if he had succeeded in what he wanted to do, is incalculable Peter Clarke Deputy Assistant Commissioner Trials give terror battle insight Questions over ricin conspiracy

In a statement, Deputy Assistant Commissioner Clarke said: "The impact on the public, if he [Bourgass] had succeeded in what he wanted to do, is incalculable."

And, paying tribute to DC Oake, he went on: "He died protecting the public from a vicious terrorist.

"It would be hard to underestimate the fear and disruption this plot could have caused across the country."

Cyanide poison

Anti-terrorist squad officers found a suspected chemical weapons laboratory when they raided a flat in Wood Green, north London, in January 2003.

They discovered castor oil beans - the raw material for ricin - along with equipment needed to produce it and recipes for ricin, cyanide, botulinum and other poisons, along with instructions for explosives.

No ricin was actually found at the flat or at any of the defendants' other addresses.

Courage of dead policeman

After the raid police launched a nationwide search to find Bourgass, who fled from London to Manchester, where he was captured on 14 January 2003.

It was there, during a desperate bid to get away, that he stabbed DC Oake to death with a kitchen knife and injured four other officers.

In June 2004 Bourgass was jailed for life for DC Oake's murder and told he must serve at least 20 years behind bars.

Reporting restrictions covering the murder conviction were lifted at the Old Bailey on Wednesday.

Police believe the failed asylum-seeker, who has claimed to be Algerian, was an al-Qaeda operative and say he had discussed various ways of spreading nicotine poison, including smearing it on car door handles in the Holloway Road area of north London.

Another man, Mohammed Meguerba, who jumped bail and fled Britain, is awaiting trial in Algeria.

On 8 April, a jury at the Old Bailey cleared Mouloud Sihali, David Khalef, Sidali Feddag and Mustapha Taleb of conspiracy to commit murder and conspiracy to cause a public nuisance.

Bourgass was convicted of conspiracy to cause a public nuisance by the use of poisons and/or explosives to cause disruption, fear or injury.

Castor oil beans are used to make ricin

The jury was discharged after failing to reach a verdict on a second count - conspiracy to commit murder.

Charges have also been dropped against four other men, Samir Asli, Khalid Alwerfeli, Mouloud Bouhrama and Kamel Merzoug, who were due to face trial next week.

Defence lawyers claimed that Sihali, Khalef, Feddag and Taleb were innocent people who were caught up in the police investigation simply because they knew Bourgass or, in the case of Taleb, had operated the photocopier on which the poison recipes were duplicated.

In his statement, Deputy Assistant Commissioner Clarke added: "We must also remember that this case was about a conspiracy between a small group of terrorists.

"I would like to make it absolutely clear, as I have in the past, that the police service knows that they are not representative of the overwhelming majority of the law-abiding Muslim community who have stated their total rejection of violence and terrorism."