Tahir Mahmud told Aljazeera.net that his family have not seen Tariq Mahmud, a British national, since he was arrested in Pakistan in October.
He said he feared Tariq was being detained with more than 600 other "terrorism" suspects in the notorious US detention camp in Cuba.
"I have no reason to believe the Pakistani authorities when they say that my brother is still in Pakistan. They have changed their story more than once, and no one from his legal team or from the British Foreign Office have seen him which leads me to believe he is in American custody in Guantanamo Bay."
Tahir Mahmud also criticised the British government for not pressing the Pakistanis on the whereabouts of his brother, and for not supplying the family with information on his detention.
"I believe that if my brother was a white British man and he had gone missing whilst under arrest in Pakistan, Tony Blair would have stepped in and demanded some answers."
And Abd al-Rahman Siddidque, Tariq Mahmud's soliciter in Pakistan, told Aljazeera.net confusion surrounded the whereabouts of his client.
Siddique said Pakistani intelligence officers (ICI) arrested Mahmud on 4 October while he was out shopping in Rawalpindi with another British national.
"I believe that if my brother was a white British man and he had gone missing whilst under arrest in Pakistan, Tony Blair would have stepped in and demanded some answers"
The British Foreign Office confirmed in November a second British man was arrested at the same time as Mahmud, but has since returned to the UK.
Mahmud's lawyer said the first Attorney General handling the case in October had been petitioned by him to ensure that Mahmud attended the court hearing.
But despite the judge also requesting Mahmud appear in court, he has failed to appear since his arrest.
A second Attorney General has since been appointed to oversee Mahmud's case, and last week denied that he had been arrested by Pakistani security officers.
"These are extraordinary circumstances that my client finds himself in. The Pakistani authorities did a complete U-turn last week, denying that Tariq Mahmud was ever arrested in Pakistan let alone charged.
"This is after nearly three months of telling his legal team, his family and the British government that they had detained him. This is a completely unacceptable situation for my client to be in," said Siddique.
However, the Pakistani lawyer returned to the court in Islamabad on Tuesday to be told that Mahmud was in fact in Pakistan, and that he was being detained in a prison "in the Punjab".
"I was told by the Attorney General this morning that my client was a threat to Pakistani state security and that the federal government of Pakistan believes Mr Mahmud has links with the al-Qaida network. I will now be pressing the authorities again for access to my client".
The lawyer added the Atorney General had presented the judge on Tuesday with an arrest order showing that Mahmud had been detained by Pakistani security officials on 10 December.
Pakistani police have stepped up
security in the country since 9/11
The order said Mahmud would be held in Pakistani detention for three months while officials investigate his alleged links to al-Qaida.
"What we are seeing now is a complete farce," said Siddique. "There is no evidence that Mahmud has any links to al-Qaida. Furthermore, the authorities first did a U- turn saying that they had never arrested Mr Mahmud.
"They now claim that he is in their detention, but only since 10 December. This means that my client has been arrested unlawfully and detained without charge for nearly three months."
Aljazeera.net contacted the Attorney General's office and the Pakistani foreign ministry, both of whom were unavailable for comment.
But the British Foreign Office has said they have been pressing for consular access to Tariq Mahmud since his detention, but admitted that no British official had seen him.
The Foreign office said Mahmud has joint British and Pakistani nationality. Under Pakistani law, this means he will be treated as a Pakistani national and British consulate access is restricted.
"We sent a note verbal (a diplomatic letter) to the Pakistani authorities on 17 December to request once again for consular access. We are following developments closely," said a Foreign Office spokesperson.
Nevertheless, Mahmud's solicitor in the UK has said it is unacceptable for the Foreign Office to sit back and simply follow the case.
Natalia Garcia said she wanted to see her client returned to the UK.
"Tariq Mahmud should be brought back to the UK without charge, the British government needs to step in and ask the Pakistani authorities to give his legal team access to him. It's outrageous that this has been going on for as long as it has," she said.
"Tariq Mahmud should be brought back to the UK without charge, the British government needs to step in and ask the Pakistani authorities to give his legal team access to him. It's outrageous that this has been going on for as long as it has"
Tariq Mahmud's UK lawyer
Aljazeera.net also contacted US Central command to ask if they could confirm if Mahmud was in their custody.
Major Pete Mitchell said: "It's our policy that US command does not talk about individuals that may or may not be in our custody. All I can say is that if this individual is in
coalition custody he will be treated well."
Tariq Mahmud, a taxi driver from Birmingham, UK, left for Pakistan two years ago with his wife and two daughters to attend to family business.
Aljazeera.net exclusively revealed in October the British national was being detained by authorities in Pakistan, who suspected him of having links to al-Qaida.
Sources said Mahmud, 30, was arrested in an international sting set up by American, British and Pakistani intelligence officials.
However, the claim has been denied by the British Foreign Office, US Central Command and the Pakistani authorities.