British forces are detaining 80 to 90 Afghan nationals in a holding facility at Camp Bastion, Defence Secretary Philip Hammond has confirmed.
UK lawyers acting for eight of the men said they had been held for up to 14 months without charge in what could amount to unlawful detention.
They compared it to when the public became aware of Guantanamo Bay and want the UK High Court to free the men.
But Mr Hammond said their release would put British troops at risk.
British forces in Afghanistan are allowed to detain suspects for 96 hours.
However, in "exceptional circumstances" to gather critical intelligence, for example - they can hold them for longer.
Documents seen by the BBC suggest 85 suspected insurgents are being held at Camp Bastion.
UK lawyers acting for eight of the men being detained said their clients were arrested by British soldiers in raids in villages in Helmand and Kandahar provinces and have been held for between eight and 14 months without charge.
But the defence secretary dismissed claims that the UK is operating a secret facility in Afghanistan as "patently absurd".
Mr Hammond told BBC Radio 4's Today programme that Parliament had been informed by this government and the previous one about the detention of Afghan suspects.
He declined to say how long some had been held and said the precise number of detainees fluctuated.
General Zahir Azimi, a spokesman for the Afghan Ministry of Defence, said the detentions were illegal and inhumane.
"The prisoners must be handed over to the Afghan authorities," he said. "After their handover to us, they will be dealt with according to our judicial laws, and the agreements reached with the international community."
Lawyers for the men, whom the BBC has chosen not to name over fears for their safety, launched habeas corpus applications at the High Court in London on 18 April, with a full hearing due in late July.
"The UK could have trained the Afghan authorities to detain people lawfully with proper standards and making sure that they are treated humanely," Phil Shiner, of Public Interest Lawyers, told the BBC.
"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."
He said the UK was acting in an "entirely unconstitutional" way and that "Parliament has not been told that we have this secret facility".
Mr Hammond told Parliament in December that British forces were "holding significant numbers of detainees" who, for legal reasons, could not "be transferred into the Afghan system".
The BBC's legal affairs correspondent Clive Coleman said the government was in a bind. He said there was a dilemma about what to do with people who are believed to be actively involved in the insurgency, whom the Army can hold for only 96 hours, and who cannot be transferred to the Afghan authorities.
In preparatory legal arguments at the High Court on 22 April, Mr Justice Collins told the government that the case raised serious questions about the British army's power to hold suspects in Afghanistan.
He said the UK could not operate a Guantanamo Bay-style prison, referring to the US facility in which suspects are being held indefinitely without trial.
A senior government lawyer, James Eadie QC, described this situation in court as a "perfect legal storm" because the Army suspects all the detainees have links to insurgents.
The families of two of the men who appear to have been held the longest said they were arrested in the spring last year and interrogated in the following weeks.
But legal papers state their interrogation ended "many months ago".
The families only established where the men were being held with the help of the International Committee of the Red Cross.
One, a teenager, has been held for 14 months, while the other, a 20-year-old father, has been held for 12 months.
In legal papers seen by the BBC, Dan Squires, a barrister for the 20-year-old, told the High Court: "He has not been granted access to lawyer nor brought before a court.
"He does not know how long he is to remain detained or for what purpose. He has asked whether he will be transferred to Afghan authorities but had been told they do not consider that he has committed any criminal offence and so do not want to receive him."
Mr Shiner said Mr Hammond had until last week refused to allow the detainees access to legal representation but had now granted lawyers an hour-long telephone call with two of the Afghans on Wednesday.
The defence secretary was keen to point out that the Afghans' case was being brought "at the expense, of course, of the British taxpayer, because Mr Shiner's actions are funded by the legal aid system".
"They are asking the court to release these people to turn them 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 lives at risk."