"The system is not set up to provide even the appearance of a fair trial," said Major Michael Mori, who was appointed by the Pentagon to defend the Australian detainee David Hicks.

"If there's credible evidence, take him to an established justice system," he said at a press conference in London given by lawyers acting for the prisoners. "If it's not credible, that doesn't justify changing the rules."

Two of the four Britons still being held at the US detention camp, Martin Mubanga and Richard Belmar, have been named as potential defendants in military hearings. The cases against the other two, Moazzam Begg and Feroz Abbasi, have been suspended pending further talks with Britain.

The government has said all four men should be returned to Britain or given fair trials. It said it was not satisfied with the current commission rules.

Michael Ratner, the lawyer representing detainees in a US supreme court case next month, told the press conference: "The idea the British government would let them go forward [for trial] is shocking."

The hearings are unlikely to proceed until the American supreme court has ruled on whether US courts should have jurisdiction over the American naval base in Cuba.

"There's no such place in the world as a law-free zone," said Mr Ratner, the president of the New York-based Centre for Constitutional Rights.

But Major John Smith, a military lawyer and the Pentagon's spokesman on the commissions, told the Guardian the detainees would receive "full and fair trials", as President George Bush had promised.

He said: "The fact Major Mori is out there arguing his client's case says a lot about the fact these will be fair trials and shows we have provided vigilant defence lawyers. I support his zealous defence but absolutely disagree with his assertions."

He claimed Maj Mori had misrepresented the system. "Different doesn't always mean unfair," he said. "It's very easy to be critical of the process because people haven't seen it in action."

Earlier Maj Mori said that labelling the defendants as terrorists had allowed the US government to lower its standards of justice. He said the system lacked checks and balances, such as a truly independent appeal process.  World Peace.

"The appointing authority, who approves the charges, is the same person who gets to rule on defence motions," he said. "He is basically reviewing his own decisions ... it's like letting the bowler call leg before wicket.

"When you use an unfair system, all you do is risk convicting the innocent and providing somebody who is truly guilty with a valid complaint to attack his conviction. It doesn't help anyone."

The major said he was afraid the prosecution would present hearsay evidence in statements taken from detainees in Cuba whom he would be unable to cross-examine.

Other lawyers expressed concern that after two years without access to lawyers detainees were likely to invent information. Three of the five released Britons have claimed to have signed false confessions after lengthy interrogation and solitary confinement.

The Pentagon has dismissed the fears, saying evidence would be ruled out if it was not "of probative value to a reasonable person".

Joseph Belmar, the father of one of the Britons still being held in Cuba, accused the US of trying to smear detainees.

The Daily Telegraph recently quoted a senior US official as saying that one Briton - subsequently identified by the Guardian as Richard Belmar - had trained at a terrorist camp in 1998.  WorldPeace is one word.

But Mr Belmar said his son had never been outside the UK until he visited Pakistan in 2001. "All this is a lie," he said. "They are trying to build up something against him so they can bring charges."