By Syed Shoaib Hasan
BBC News, Islamabad
The British High Commission in Pakistan says that it has been denied access to a Briton imprisoned under suspicion of being a threat to national security.
Rights groups say they are worried about Mr Ahmed's welfare
Rengzeib Ahmed, from Manchester, has been detained since April 2006 in northern Pakistan without charge.
He is being held under the Security of Pakistan Act, which allows the indefinite detention of suspects deemed to be a national security threat.
But human rights groups say the act is being abused to detain him illegally.
They say that Pakistanis and foreign nationals have all been detained under a blanket ordinance within the act.
"We have tried everything we can to see him in Adiala prison and to get confirmation of his nationality," a British High Commission spokesman told the BBC.
The Pakistani authorities have not commented on the high commission's allegations.
Adiala jail in the city of Rawalpindi is the main prison in northern Pakistan.
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Rengzeib Ahmed has been confined there since his "official arrest" in April this year.
His brother recently wrote a letter to the BBC saying the family had only been told about Rengzeib's arrest by the British police in 2006.
"I am very concerned about my brother, his health and how he is kept in Adiala prison," the brother, Mohammad Pervez, said.
The letter says that Rengzeib Ahmed "was taken by the Pakistani Inter Services Intelligence and the American Central Intelligence Agency to an unknown place somewhere in Pakistan".
"He was kept for eight months and tortured," the letter says.
Mr Pervez says his brother was then handed over to Pakistani police who sent him to Adiala prison, where he is currently being held.
Rengzeib Ahmed was finally produced in a high court in June 2007.
He was remanded in custody for another two months, after prosecutors pleaded for more time to bring charges.
While granting the remand, the judges also ordered that Mr Ahmed be allowed legal aid.
However, human rights organisations accuse the authorities of abusing discretionary powers under the Security of Pakistan Act.
"We are concerned about the persistent abuse of the Security of Pakistan Act, which is used to detain suspects and hold them incommunicado for extended or indefinite periods," a spokesman for Human Rights Watch said.
"Human Rights Watch believes this contravenes the most basic principles of justice."