Former Lieutenant-Colonel Darrel Vandeveld told the BBC in his first interview since resigning earlier this year that the way detainees were treated at the camp on Cuba was "appalling, wrong, unethical and finally, immoral".

"I was convinced that it was impossible to guarantee that they would get a fair trial," he told the British broadcaster in the interview aired on Tuesday.

Vandeveld was a reservist called up as a military lawyer after the September 11 attacks on the United States, and served in Iraq, Bosnia and Afghanistan.

In 2007, he became a prosecutor for the military commissions set up to try suspects in the US 'war on terror' who were being held at Guantanamo. He said he approached the job with enthusiasm and "zero doubts".

When he arrived, however, he found the prosecutor's office in "disarray", and defence lawyers were not being given evidence that could help their clients -- sometimes evidence that they had been "mistreated" to obtain a confession.

Unable to share his doubts with friends and family, the devout Catholic emailed his concerns to peace activist and Jesuit priest Father John Dear. Dear replied telling him to quit.

Vandeveld said he regarded that time as "the most anguished period of my life", but now had to speak out.

"I know so many fighting men and women whose honours have been stained by the taint of Guantanamo, and they don't deserve that," he said.

"So I'm here to tell the truth about Guantanamo and how it involves a few people... who have sullied America's military and who have sullied our constitution, and that is unacceptable. It makes me angry."

In response, a Pentagon spokesman told the BBC, "We dispute Darrel Vandeveld's assertions and maintain the military commission process provides full and fair trials to accused unlawful enemy combatants who are charged with a variety of war crimes."

The special tribunals were created to prosecute the alleged perpetrators of the September 11, 2001 terror attacks. Comprised of a judge and a military jury, they bear little resemblance to the federal and court martial systems.

Although US president-elect Barack Obama has said he is eager to end the tribunals, what to do with the detainees housed at Guantanamo is one of the thorniest issues he will have to address early in his presidency.

One of the suspects Vandeveld was prosecuting was Mohammed Jawad, who was arrested as a teenager in Kabul in 2002. He faces trial on January 5.