`I was assaulted three times by Guantanamo riot squad'

Martin Mubanga, the British Guantanamo Bay detainee released a fortnight ago, is to sue the Government after alleging that UK intelligence officials colluded in his arrest and transportation to the American-run camp for suspected terrorists.

Mr Mubanga claims the reasons given for his 33-month detention - that he was travelling to Zambia on false documents and was involved in planning terrorist attacks - were fabricated after intelligence officers failed to link him with a suspected Taliban fighter who had assumed his identity.

He makes the allegations in an interview with today's Observer, which includes a graphic account of the treatment he says he endured. On one occasion at Guantanamo, he was forced to urinate in the corner of a room while bound hand and foot. The interrogating officer then used a mop to daub him in his urine while taunting him with racist insults.

The claims will add to concerns over plans announced by Charles Clarke, the Home Secretary, to detain suspected terrorists without charge. Mr Mubanga, 32, was released last month along with three other British detainees, Moazzam Begg, Feroz Abbasi and Richard Belmar. After police interrogators at Paddington Green station, London, failed to find any evidence of terrorist activity, all four were set free unconditionally. This is the first time that any of them have spoken publicly about their ordeal, although legal testimony from Mr Begg, revealed last week in The Independent on Sunday, has also alleged torture and assault at the hands of Guantanamo guards.

Mr Mubanga's solicitor, Louise Christian, said yesterday: "We are hoping to issue proceedings for the misfeasance of [UK] officials who colluded with the Americans in effectively kidnapping him and taking him to Guantanamo."

Mr Mubanga, who holds British and Zambian nationalities and has always denied any involvement with al-Qa'ida, says he went to Afghanistan in 2001 to study Islam. After the war against the Taliban began he fled to Pakistan and, having lost his British passport, his family sent him his Zambian passport, which he used to travel to the African state to meet relatives. He was arrested there in March 2002 shortly after reports that a man sharing the same name had been fighting with the Taliban.

Mr Mubanga says that initially he was interrogated in guarded hotel rooms by Zambian officers, who he claims asked him to choose which nationality he wanted to assume. He answered "British". Soon afterwards, a female American official and a British man calling himself "Martin" who said he was from MI6, arrived and started interrogating him.

A few days later, Mr Mubanga was taken to a military airstrip and flown to the US detention camp in Cuba.

At the camp, he was subjected to the strictest prison regime, known as "BI [basic item] loss" - his blankets, towels and all his clothes apart from his underpants were taken from him. And shortly before his release, he says he was assaulted three times by Guantanamo's "Instant Reaction Force" riot squad.

A Home Office spokesman refused to comment on the activities of the British security services.