The United States Government says it will not let a man charged in connection with the 11 September attacks question an alleged al-Qaeda leader.
Prosecutors say Moussaoui was to have been the 20th hijacker
This is despite a court ruling allowing the interview.
Defendent Zacarias Moussaoui, who is acting as his own attorney, argues that allowing him to interview al-Qaeda suspect Ramzi Binalshibh is crucial to proving his own innocence.
The Virginia judge handling the case agreed to the interview in principle but the prosecution is contesting her decision on the grounds of protecting the "nation's security at a time of war".
The judge concluded that a satellite link-up interview, to be played during a trial, could shed light on Mr Moussaoui's contention that he was not part of the 11 September conspiracy.
But on Monday federal prosecutors reaffirmed that they will not produce Mr Binalshibh for questioning under any circumstances.
They acknowledge that this may lead to the case being dismissed.
US Attorney Paul McNulty said the interview with Mr Binalshibh would "necessarily result in the unauthorised disclosure of classified information".
"Such a scenario is unacceptable to the government, which not only carries the responsibility of prosecuting the defendant, but
also of protecting this nation's security at a time of war with an enemy who has already murdered thousands of our citizens."
The prosecutors also asked the judge to postpone any action pending a ruling by a higher court on their appeal.
It is thought that if the government is forced to allow Mr Moussaoui access to Mr Binalshibh, it may decide to drop the criminal charges and try him before a military tribunal.
Mr Moussaoui, a French national of Moroccan origin, is the only person in America charged in connection with attacks on the World Trade Center and the Pentagon on 11 September 2001.
The indictment charges that - like the hijackers - he was an al-Qaeda loyalist who had bought flight manuals and cockpit videos.
Ramzi Binalshibh is being held at a secret location
He is alleged to have received funds from Ramzi Binalshibh, an alleged al-Qaeda operative in Germany who is also believed to have funded the hijackers.
Prosecutors say Mr Moussaoui would have been the 20th hijacker, had he not been arrested in August 2001 - when the flying lessons he was taking in Minnesota aroused suspicion.
Mr Moussaoui admits to being a member of al-Qaeda, but denies being part of the 11 September attacks.
If found guilty he could face the death penalty.
Mr Binalshibh was himself captured last September in Pakistan, and is being held at a secret location.