The filing came hours ahead of a hearing before US District Judge Leonie Brinkema, who earlier in the week met Moussaoui and determined he was competent to enter such a plea.
In the latest twist in a case that has been full of them, Moussaoui's lawyers filed papers under seal at the federal courthouse in Alexandria, Virginia, titled, "Sealed Suggestion of Defense Counsel as to Defendant's Incompetence to Plead Guilty and for a Sentence of Death".
Moussaoui's lawyers declined to comment but previously had said such a filing was planned.
Moussaoui, a French citizen, is the only person charged in the United States in connection with the 11 September 2001 attacks that killed nearly 3,000 people.
The hearing was scheduled after Moussaoui sent Brinkema a letter saying he wanted to plead guilty.
Moussaoui is an accused in the
11 September attacks
The mercurial Moussaoui still could change his mind about pleading guilty, which he did once before.
Picked up in August 2001 after arousing suspicion at a Minnesota flight school, Moussaoui was transformed from an immigration violator into a terrorist defendant three months after the 11 September attacks.
Moussaoui's criminal trial was to have been an evidentiary showcase of al-Qaida. Those plans would be scuttled by Friday's scheduled guilty plea to a six-count indictment.
Jumping past a trial, the lawyers Moussaoui tried to fire would eventually be defending him in a penalty phase proceeding, a sort of mini-trial before 12 jurors who will decide whether to spare his life.
Some legal experts say Moussaoui's decisions seem to make no sense, unless he wants to die.
Moussaoui is charged with conspiracy to commit acts of terrorism, commit aircraft piracy, destroy aircraft, murder government employees and destroy property. The first four charges carry a maximum sentence of death.