A US federal judge has ruled that prosecutors may not seek the death penalty for the only person in the US charged over the 11 September attacks.
Mr Moussaoui was arrested at a flight school in Minnesota
District Judge Leonie Brinkema did not agree to drop all charges against
Zacarias Moussaoui - but she barred prosecutors from arguing that he had been involved in the attacks.
The prosecution of Mr Moussaoui has been dogged by a row over his request to interview other al-Qaeda suspects being held in US custody, who he says would prove his innocence.
The government has refused to allow him access to the detainees on grounds of national security, a fact that the judge said could prejudice his trial.
Mr Moussaoui, a 35-year-old French citizen of Moroccan origin, was indicted 21 months ago on four counts of conspiracy to commit terrorism, which carry the death penalty.
He had been under arrest on immigration charges when hijackers crashed civilian airliners into the World Trade Center and other targets, killing more than 2,800 people.
Judge Brinkema said government prosecutors could not argue that the defendant was involved in, or had knowledge of, planning the attacks.
She postponed the effect of her ruling so the government could appeal.
The BBC's Jon Leyne in Washington says that even if Mr Moussaoui is cleared by the civilian court, he could still be turned over to a military one.
Mr Moussaoui and his court-appointed defence team had been seeking the full dismissal of the charges.
Ironically, the government had also asked for their dismissal, believing that this would be the quickest route to intervention by an appellate court.
Mr Moussaoui had been seeking to interview three major al-Qaeda suspects who are being held by the US at undisclosed locations:
Khalid Shaikh Mohammed, alleged to be the 9/11 mastermind;
Ramzi Binalshibh, believed to be a key planner of the attacks;
Mustafa Ahmed al-Hawsawi, believed to be an al-Qaeda financier.
Judge Brinkema ruled on Thursday that the US authorities were denying the defendant the right to important witnesses.
"The United States may not maintain this capital
prosecution while simultaneously refusing to produce witnesses
who could, at a minimum, help the defendant avoid a sentence of
death," she wrote.
"It would simply be unfair to require Moussaoui to defend
himself against such prejudicial accusations while being denied
the ability to present testimony from witnesses who could
assist him in contradicting those accusations."
Prosecutors have argued that Mr Moussaoui's actions mirror those of the 19 hijackers behind the 11 September attacks.
He went through flight instruction, is alleged to have received money from a terrorist suspect and is alleged to have been trained at an al-Qaeda camp.
However, the US authorities have not offered evidence of a direct link between Mr Moussaoui and the hijackings.
He has said in court that he provided a guest house for al-Qaeda members, but insisted that he was not involved in the attacks.
Mr Moussaoui told the court he was a member of al-Qaeda and had sworn a pledge of allegiance to Osama Bin Laden but he later withdrew the remarks.