Defence Secretary Philip Hammond confirms up to 90 Afghans are being detained at Camp Bastion, but the government hopes to transfer them to Afghan custody in the near future.
British lawyers for eight of the men say their detention at a “secret” site in Afghanistan could be illegal, and that they have been held for up to 14 months without charge.
Mr Hammond told Channel 4 News he halted their transfer to Afghan custody last year because he was worried about “abuse” at one Afghan facility. But he said he hoped to make progress “in the next few days”.
Mr Hammond told the BBC Today programme: “What I will say is that the assertion that this is a secret facility is patently ridiculous.”
He put the figure for the total number of men held “in the high tens, around 80, 90”, and said that parliamentary archives showed that the government had been transparent about the facility.
He added: “These are people suspected of murdering British troops, facilitating or planting or being involved with IEDs (improvised explosive devices) at a time when most people are focused on how we protect our troops from being murdered, whether it’s on the streets of London or on the battlefield.”
But Phil Shiner, of Public Interest Lawyers, told Today that the Ministry of Defence (MoD) had refused him access to his clients and it was only as a result of legal proceedings that telephone contact had been established.
“What happens is the UK could have trained the Afghan authorities to detain people lawfully with proper standards and making sure that they are treated humanely,” he said.
“They could have then monitored that, including with ad hoc inspections, to make sure that the Afghans were obeying the law. They have chosen not to do so.
“And they have chosen to go down a route which I think is completely worrying and entirely unconstitutional, where no one’s been told, parliament has not been told that we have this secret facility.”
Philip Hammond said lawyers for the men wanted them released “back to the battlefield so they can carry on with the activities for which they were detained in the first place, putting British troops and other Isaf (International Security Assistance Force troops) lives at risk”.
The defence secretary conceded that the number of men held was not ideal and usually no more than 20 would be detained at any one time.
“Of course, these numbers that we currently have are far higher than traditionally we would expect to have because the system effectively has been blocked up by the problems with transfer into the Afghan system.”
He said Britain was “working with the Afghans and with other allies to develop a safe pathway for the transfer of these detainees into the Afghan judicial system”.
In December 2012, the British government announced it was halting the transfer of suspected insurgents to the Afghan authorities after claims that detainees had been tortured by Afghan interrogators.
James Eadie QC, acting for the government, told the high court transfers would end because of “the real risk of either serious mistreatment or a flagrant denial of justice”.
But Afghan President Hamid Karzai has said that all prisoners should be handed over to Afghan forces.
According to the MoD, British forces in Afghanistan are allowed to detain suspects for 96 hours, but can hold them for longer in “exceptional circumstances”.
The men were detained in raids in villages in Helmand and Kandahar provinces. Lawyers have now launched court proceedings in London on their behalf.