A legal charity is suing the Government in connection with the extraordinary rendition of two men arrested in Iraq, passed to the US authorities and held at a detention centre in Afghanistan for the past five years.

A US Airforce Chinook helicopter (C) prepares for take off at the Bagram Airbase, north of Kabul. John Hutton, then UK Defence Secretary, admitted in February that British forces in Iraq handed over two terrorist suspects to the US, which flew them to Bagram, a notorious detention centre outside Kabul.(AFP/Manan Vatsyayana) The legal challenge aims to force Britain to reveal the identity of the men - believed to be from Pakistan - and the whereabouts of their families to enable the charity Reprieve to obtain authorisation to challenge their detention.

Clive Stafford Smith, director of Reprieve, said in a statement today that at least one of the detainees is suffering from serious mental problems because of alleged mistreatment at Bagram air force base.

"These two men have been held in appalling conditions for five years, and for all that time the British Government chose to do nothing," he said.

"We have an urgent moral, as well as legal, duty to repair the damage his rendition has caused. How many more times is the Government going to say one thing - that they never cover up complicity in torture - while doing the opposite?"

The Government has admitted its involvement in extraordinary rendition but failed to give the two men their legal rights, according to Mr Stafford Smith.

"Imagine, if you will, that a criminal was to apologise for taking part in a kidnap, but then refuse to name his victims, or to help to secure their freedom. We would hardly accept the apology as being sincere," he said.

Reprieve also released comments from Omar Deghayes, a former inmate at Bagram, to highlight the alleged ill treatment suffered by detainees.

"Lying on the floor of the compound, all night I would hear the screams of others in the rooms above us as they were tortured and interrogated," Mr Deghayes said.

"My number would be called out, and I would have to go to the gate. They chained me and put a bag over my head, dragging me off for my own turn. They would force me to my knees for questioning, and threaten me with more torture."

John Hutton, then Defence Secretary, admitted in February that British forces in Iraq handed over two terrorist suspects to the US, which flew them to Bagram, a notorious detention centre outside Kabul.

He told MPs that the men were members of Lashkar-e-Taiba, a proscribed organisation with links to al-Qaeda.

The admission was an embarrassment for the Government, which has denied that it played any part in helping the US with the process of extraordinary rendition. The procedure, condemned by human rights groups, has allegedly led to terror suspects being transferred by the United States to third countries where torture is not illegal.

The two British men were arrested in Iraq in early 2004 and transported to Afghanistan in June of that year, according to Reprieve. They have been held without charge at Bagram ever since.

Reprieve said that it wrote to Mr Hutton immediately after his statement to Parliament, asking for information about the two men in order to identify them, locate their families and mount a challenge to their ongoing.

Three months later, it said, the Ministry of Defence responded by saying that the men's identity was secret and that it would violate their rights under the Data Protection Act to reveal this information.

Interviews with former detainees enabled the charity tentatively to identify the pair as Sala el Din and Saifullah. But the information is insufficient to begin litigation.

"We are now suing the Government to force them to reveal the necessary information," Reprieve added. The charity is holding a press conference this afternoon about the case.

A comment was not immediately available from the Ministry of Defence.