Speaking to journalists in Australia on Tuesday, Major Michael Mori said David Hicks was facing a military tribunal rigged to get convictions in which there was no right of an independent appeal.
"Everyone is so emotionally charged by September 11 that they are eager to label people a terrorist without using an established justice system to determine innocence or guilt."
Hicks is one of the few detainees at Guantanamo to have been appointed a legal defence team, although there are over 660 men held without charge in the Cuba prison camp.
Tribunals and trials
Military tribunals were set up by the Pentagon to try non-US citizens captured during the US-led 'war on terror'.
However, Washington is planning to conduct trials before the tribunals, formally called military commissions, of some of the 660 prisoners held in cages at Guantanamo.
Mori said it was outrageous the military commission process was set up with no independent checks and balances.
He concluded the commissions were designed to convict rather than determine guilt or innocence.
The Australian government has acknowledged the 28-year-old Hicks, who was arrested in Afghanistan in late 2001, will be among the first group to face the tribunal.
But authorities in Canberra have made no attempt to bring him back to Australia where laws against terrorism only date back to 2002.
"Everyone is so emotionally charged by September 11 that they are eager to label people a terrorist without using an established justice system to determine innocence or guilt"
Major Michael Mori,
US Marine Corps defence lawyer
Mori also said Hicks had broken no Australian law, no law in Afghanistan and had not harmed any Americans.
He questioned how non-Americans could be subjected to US law.
"The military commission process is an unfair system."
Explaining the unexplainable
Mori, who has met Hicks five times in recent months, said it was very hard to tell someone who had been locked up for two years that he was about to face an unfair system and his country would allow that to happen.
Other countries, such as Spain, Denmark, and Saudi Arabia, have successfully argued for their citizens in Guantanamo to be returned home to face possible prosecution.
Hicks has yet to face any charges, although Australian Attorney-General Philip Ruddock has said the US indicated charges would centre on suspected training with al-Qaida.
Mori said it was still unclear exactly what charges would be laid or when Hicks would face the military tribunal.
He is lobbying to change the system so Hicks faces a conventional US military court martial, but he was making little headway enlisting support from Australia and he has no plans for a meeting with government officials there.
US officials have promised no Australian will face the death penalty.