The whistleblower, an army major inside the military court system the US has established at Guantanamo Bay, has described the detention of one prisoner, a hospital administrator from Sudan, as "unconscionable".

His critique will be the centrepiece of a hearing on 5 December before the US Supreme Court when another attempt is made to shut the prison down. So nervous is the Bush administration of the latest attack . . . and another Supreme Court ruling against it . . . that it is preparing a whole new system of military courts to deal with those still imprisoned.

The whistleblower's testimony is the most serious attack to date on the military panels, which were meant to give a fig-leaf of legitimacy to the interrogation and detention policies at Guantanamo Bay.

The major has taken part in 49 status review panels.

"It's a kangaroo court system and completely corrupt, " said Michael Ratner, the president of the Centre for Constitutional Rights, which is coordinating investigations and appeals lawsuits against the government by 1,000 lawyers. "Stalin had show trials but at Guantanamo they are not even show trials as it all takes place in secret."

Combatant status-review tribunals were held for 558 detainees at Guantanamo in 2004 and 2005. All but 38 detainees were determined to be "enemy combatants" who could be held indefinitely without charges. Detainees were not represented by a lawyer and had no access to evidence.

The only witnesses they could call were other so-called "enemy combatants".

The major has said in the rare circumstances in which it was decided detainees were no longer enemy combatants, senior commanders ordered another panel to reverse the decision. He also described "acrimony" during a "heated conference" call from Admiral McGarragh, who reports to the secretary of the US Navy, when a panel refused to describe several Uighur detainees as enemy combatants. Senior military commanders wanted to know why some panels considering the same evidence would come to different findings on the Uighurs, members of a Muslim minority in China.

When the whistleblower suggested over the phone that inconsistent results were "good for the system. . . and would show that the system was working correctly", McGarragh, he said, had no response. The latest criticism emerged when lawyers investigating the case of a Sudanese hospital administrator, Adel Hamad, who has been held for five years, came across a "stunning" sworn statement from a member of the military panel. The officer they interviewed was so frightened of retaliation from the military that they would not allow their name to be used in the statement, nor even reveal their gender.

Two other military lawyers have also gone public. In June, lieutenant colonel Stephen Abraham, a 26-year veteran in US military intelligence, became the first insider to publicly fault the proceedings. In May last year, lieutenant commander Matthew Diaz was sentenced to six months in prison and dismissed from the military after sending the names of all 551 men at the prison to a human rights group.

William Teesdale, a Britishborn lawyer investigating Adel Hamad's case, said he was certain of his client's innocence, having tracked down doctors who worked with him at an Afghan hospital. "Mr Hamad is an innocent man, and he is not the only one in Guantanamo."