UK forces are breaking the law by detaining up to 85 Afghans without charge at Camp Bastion in Afghanistan's Helmand province, a British firm representing eight of the men said.
Phil Shiner, of Public Interest Lawyers, said the Afghans had been held for up to 14 months without access to lawyers or being told why they were being detained.
"It's not an answer to the prohibition on transfer when there is a real risk of torture and then in effect covertly detain up to 90 men," Shiner told Al Jazeera on Wednesday, rejecting the suggestion that the lawyers are to blame for blocking the prisoner transfers.
"It's indefinite detention. They're not allowed access to their lawyers and that's plainly unlawful and clearly unacceptable," he added.
He said he had called on the minister of defence to explain why the detainees had been refused access to legal representation for so long.
"People have been detained indefinitely, incommunicado," he said. "Until today, their relatives have not been able to see them."
He said three high court judges would listen to the ministry's arguments in July at a four-day hearing.
'Legal under UN mandate'
The Ministry of Defence denied their detention was illegal, and said the threat of court action in Britain was preventing the transfer of the detainees to Afghan authorities.
Afghan President Hamid Karzai's spokesman Aimal Faizi said the government was investigating the Camp Bastion detentions.
Last November, British Defence Secretary Philip Hammond imposed a ban on transferring suspects to Afghan forces due to concerns over ill treatment, a move he said was pushed by the very law firm representing the Afghan detainees at Camp Bastion.
The UK's ministry of defence said in a statement that all temporary UK facilities at camp Bastion were regularly monitored by the International Committee of the Red Cross and that all UK detentions in Afghanistan were legal under the UN mandate. They comply with all applicable international obligations, the ministry said.
Hammond said the detainees were suspected of murdering British troops and planting roadside bombs.
Housing around 30,000 staff, Camp Bastion is the largest British military base in Afghanistan. British troops are handing over security operations to Afghan forces and are due to end combat operations in Afghanistan by the end of next year.