Two former CIA contractors who designed the harsh interrogation program used after the September 11 attacks are being summoned to testify before the military tribunal at the US base at Guantanamo Bay, Cuba.

Psychologists James Mitchell and Bruce Jessen were among a dozen approved witnesses listed in a letter sent Monday by prosecutors to defense lawyers for five men charged in the 2001 attacks.

Defense lawyers in the long-running September 11 military tribunal want to question Mitchell and Jessen as part of an effort to exclude statements the defendants made to the FBI at Guantanamo after being subjected to brutal treatment in clandestine CIA detention facilities.

Two former CIA contractors have been summoned to testify over the "enhanced interrogation" program they developed used on prisoners. (AP/AAP)

The defense lawyers are also seeking to compel testimony from dozens of current and former CIA officers who were involved with what the government called the "enhanced" interrogation program.

Mitchell and Jessen gave depositions in a civil lawsuit filed by the American Civil Liberties Union on behalf of three former US prisoners, including one who died in custody.

That case was settled for undisclosed terms in August 2017 and the two former contractors did not testify in court.

Five Guantanamo Bay prisoners are facing trial over their alleged role in the September 11 attacks. (AP/AAP)

"This will be the first time Dr Mitchell and Dr Jessen will have to testify in a criminal proceeding about the torture program they implemented," said James Connell, a lawyer for Ammar al Baluchi, one of the five Guantanamo prisoners facing trial by military commission for their alleged roles in the attack.

Mitchell and Jessen helped design an interrogation program that included such abusive techniques as prolonged sleep deprivation, confinement in small, enclosed spaces and waterboarding.

The former contractors have defended their work, arguing it was legal and necessary.

Protesters perform a live waterboarding demonstration outside the Spanish Consulate in Manhattan to urge prosecution in Spain of the alleged involvement of Bush administration officials in the torture of terror suspects, in 2009. (AP/AAP)

A Senate investigation in 2014 found that Mitchell and Jessen's techniques were not effective.

At the earliest, the two former contractors would testify at a pretrial hearing scheduled for July, though it could be later.

The proceedings have faced repeated delays, largely because of legal issues related to the treatment of the five defendants while in CIA custody.

Khalid Shaikh Mohammed, the alleged September 11 mastermind, photographed shortly after his capture in Pakistan in 2003. (AP/AAP)

The defendants include Khalid Shaikh Mohammad, who has portrayed himself as the mastermind of the September 11, 2001, hijacking plot.

He and his co-defendants were arraigned in May 2012 on charges that include terrorism and nearly 3,000 counts of murder in violation of the law of war.