In one of the emails obtained by the Australian Broadcasting Corporation, prosecutor Major Robert Preston wrote to his supervisor in March last year that the process was perpetrating a fraud on the American people.

"I consider the insistence on pressing ahead with cases that would be marginal even if properly prepared to be a severe threat to the reputation of the military justice system and even a fraud on the American people," Preston wrote, according to the ABC.

"Surely they don't expect that this fairly half-assessed effort is all that we have been able to put together after all this time."

Of the 510 detainees being held at the Guantanamo Bay US naval base in Cuba, most of them captured during the US attack on Afghanistan in late 2001, a dozen have been declared eligible to be charged before the military commissions.

One of those facing trial is Australian David Hicks, who was allegedly fighting for the former Taliban rulers when he was captured in Afghanistan.