Moussaoui has denied any part
of the 11 September attacks
Government lawyers who brought the case to a federal appeals court in Richmond, Virginia will argue against allowing access to Ramzi bin Alshaibah, who was captured in Pakistan last year.
He is accused of providing funding for the hijackers and co-ordinating the attacks.
The hearing on Tuesday could determine whether the trial continues in a civilian court. Moussaoui who has been representing himself in court will not be present, but his court-appointed attorneys will argue on his behalf.
The arguments will be open to the public for 40 minutes, but a one-hour closed-door session will deal with classified information.
The government wants the appeals court to overturn a January ruling by US District Court Judge, Leonie Brinkema, who said Moussaoui and his lawyers could question Bin Alshaibah.
Moussaoui faces the death penalty and says Bin Alshaibah can help prove that he was not involved in the hijackings.
Witness could help
Brinkema ruled in January that Moussaoui and the court-appointed “stand-by” lawyers named to defend him could question Bin Alshaibah by video conference.
The government argues Brinkema does not have the right to allow access to Bin Alshaibah, saying it could jeopardise national security.
But Moussaoui and his attorneys say he has the right to speak to witnesses who can help his defence.
Moussaoui, a French citizen of Moroccan descent, was being held on immigration violations when the hijackings occurred. He has denied being part of the 11 September attacks but says he is a member of al-Qaeda.