By Syed Shoaib Hasan
BBC News, Islamabad
A man describing himself as British has been held illegally and tortured in Pakistan since August, campaigners say.
Hundreds of Pakistanis have disappeared in recent years
The man, named as Rengzeib Ahmed, says he was interrogated by British and US security agents, according to the Defence of Human Rights group.
Britain's High Commission has confirmed that Mr Ahmed is in Adiala jail in the northern city of Rawalpindi.
But a spokesman said Pakistan had yet to clarify whether he was a UK citizen or held dual nationality.
"If he is a British national we will provide all possible assistance," Aidan Liddle, head of public affairs at the High Commission, told the BBC.
"But if he's a dual national our hands are tied."
Mr Ahmed, campaigners say, maintains he is solely a British citizen.
Mr Khawaja has himself been detained a number of times
He says he is a resident of Manchester and holds a valid British passport with a Pakistan visa.
The BBC has seen a copy of his passport, number 103633059. It was issued in 2002 and gives Mr Ahmed's year of birth as 1975.
According to campaigners, Mr Ahmed has been seeking help since he was detained 10 months ago.
"They have not even allowed him access to a lawyer," says Khalid Khawaja, chief co-ordinator of Defence of Human Rights.
According to Mr Khawaja, Mr Ahmed was arrested by men in plain clothes while travelling on a bus in northern Pakistan.
He was then taken to Islamabad and interrogated for six days.
During this time, Mr Ahmed says he was tortured and kept awake. He also says he was interrogated by British and US security agents.
According to Mr Khawaja, Mr Ahmed told the British agents he had rights as a British citizen.
He was kept in confinement in the capital before being shifted to Adiala jail last month, Mr Khawaja said, adding that he had taken the case to the high court in Rawalpindi.
"The British government appear to be stalling on the matter," Mr Khawaja said.
He added that Mr Ahmed's family in the UK had been desperately trying to secure his release.
The British High Commission says it is aware of the case, is in touch with the Pakistani authorities and is awaiting a reply from them.