The Home Office today lost its legal bid to send a foreign terror suspect currently under house arrest back to Belmarsh maximum security prison.

Lawyers for the Government claimed that bail conditions were breached when two men spent 20 minutes with the 35-year-old Algerian suspect, thought to have links with al-Qaeda, in an unauthorised visit to his home on November 12, 2004.

But Mr Justice Collins, sitting at the Special Immigration Appeals Commission, ruled that the Home Secretary had not proved "to the necessary standard" that there had been a breach.

The detainee, identified only as G, is the only man held under the Government’s controversial anti-terrorist powers to be kept under house arrest instead of prison.

Mr Justice Collins said: "We are not satisfied to the necessary standard that the Secretary of State has proved a breach and in the circumstances we will take no action towards the revocation of bail."

He said the tribunal was giving G the benefit of the doubt despite evidence from the closed session leaving them with "the gravest of suspicions" that there had been unauthorised visitors.

He said: "Grave suspicion is insufficient to prove there was a breach and we are not satisfied to the necessary standard that the Secretary of State has proved a breach. In the circumstances, we will take no action towards the revocation of bail."

G was arrested in December 2001 and detained without trial until April, when he suffered a mental breakdown and was released from Belmarsh under strict bail conditions.

The Government today maintained that the alleged unauthorised visit was a very serious breach of those conditions.

Ian Burnett, QC for the Home Office, said: "At about 1300 hours on the 12 November 2004, two males, including a man who was a youth, visited Mr G at his home address.

"The visit lasted approximately 20 minutes. No prior authorisation was obtained and no subsequent authorisation was sought."

G, who attended the hearing with his wife, is not allowed to meet anybody apart from his family, lawyers and doctors and is banned from using the telephone or computer to prevent him from associating with terrorists. He must also check in with the authorities five times a day.

The detainee and his wife "roundly" denied any breach of the conditions, according to Ben Emmerson QC, for G.

Due to the secretive nature of the evidence being used against G, his lawyers were only able to guess whether the supposed visit was noticed by a human watcher, or picked up by camera or bugging surveillance, all of which might have been fallible. Mr Emmerson also said that no evidence had been provided of who the men were, the purpose of the alleged visit, or whether they entered G's home.

Mr Emmerson said that G has been scrupulous in adhering to the "incredibly onerous" bail conditions. He said: "He sleeps with an alarm clock in every room. He has to check in and he keeps a meticulous record of each time he checks in. He is acutely concerned about a return to Belmarsh. If there were to be one, his psychotic condition would deteriorate."

Siac allowed G to be released on bail on April 22, 2004, on strict conditions. But the panel said in July last year: "In granting bail, the commission did not revise its view as to the strength of the grounds for believing that he was an international terrorist and a threat to national security.

"A number of his contacts remain at large including some who are regarded as actively involved in terrorist planning.

"There is nothing to suggest that his mental illness has diminished his commitment to the extremist Islamic cause; he has the experience and capacity to involve himself once more in extremist activity. The bail restraints on him are essential."

Charles Clarke, the Home Secretary, issued a statement after the ruling. It said: "Siac, in handing down bail to G in April last year, said that any breach of his conditions would leave the court with little choice than to return him to detention.

"I am disappointed with today’s decision by the court to allow G to remain on bail. Every step will be taken to continue to monitor G’s bail conditions closely so that any further breach can be dealt with swiftly and appropriately to ensure public safety."

Last Monday, Siac approved the release of another detainee, Abu Rideh, because of mental health fears. His bail conditions are yet to be agreed.

The following day, the Home Secretary ordered the release of another detainee, known as C, but refused to explain the reasons behind his decision.

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