Mohammed Meguerba, 37, who trained in terrorist camps in Afghanistan, was held in a secret detention centre for 17 months by the Algerian intelligence service (DRS). His relatives have told The Times they were unaware he had been held from December 2002 until he was moved to Sakardji prison in Algiers last year.

When they visited him there, Mr Meguerba weighed 5st 7lb (35kg) and told his family he had been repeatedly abused. “We did not know where he was for such a long time and when we finally saw him he told us that he had been badly tortured,” one relative said.

The allegation that Mr Meguerba was tortured will bolster the asylum claims of five Algerians cleared of involvement in the ricin plot. Since their acquittals, the men have made fresh applications to remain in Britain. The Algerians, and a Libyan man, were remanded into immigration detention at the end of the Old Bailey case last month but have since been freed on bail. They are not subject to terrorist control orders.

The Foreign and Commonwealth Office has been in talks with Algeria about drawing up an agreement under which Algerian citizens could be returned there from Britain. But lawyers say that in the light of Mr Meguerba’s claims it will be impossible to deport any of the ricin defendants despite them being cleared.

After his al-Qaeda training, Mr Meguerba arrived in London in March 2002 and went to Finsbury Park mosque. He began to work with Kamel Bourgass to try to make crude poisons. In September 2002 Mr Meguerba was arrested in London during an operation into suspected terrorist fundraising. Released on bail he fled Britain and returned to Algeria, where he was arrested.

His claims of torture have been supported by an Algerian man, now living in Britain, who was detained in Algiers in January 2003 and placed by his interrogators in a room with Mr Meguerba. The man has stated that Mr Meguerba’s face was bruised, cut and swollen.

Appearing in an Algiers court earlier this month, charged with membership of a terrorist organisation, Mr Meguerba appeared frail, and with many of his teeth missing.

The allegations of torture in Mr Meguerba’s case illustrate the dilemma facing Britain and other Western democracies in fighting the war on terrorism. His “confession”, presented as 27 pages of memos written by DRS agents, led Scotland Yard to the flat in Wood Green, North London, where Bourgass was trying to make ricin and other poisons.

Bourgass, 32, an illegal immigrant, was convicted of the poison plot and is also serving life for the murder of Detective Constable Stephen Oake in Manchester in January 2003.

The Meguerba memos were not relied on by the prosecution in the ricin trial because they could have raised allegations of torture. One source close to the case said: “The Government has introduced the Human Rights Act but finds itself relying on regimes with appalling human rights records for information.” Algerian secret services denied that Mr Meguerba had been abused. Mr Meguerba remains in custody and the case against him, which contains no reference to the British ricin plot, has been adjourned indefinitely.

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