Zacarias Moussaoui had challenged the validity of his original guilty plea
A US appeals court has upheld the conviction and sentence of 9/11 conspirator Zacarias Moussaoui.
The only person charged in the US over the attacks, Moussaoui had originally pleaded guilty to conspiracy.
In 2006 he was sentenced to life in prison for his role in planning the attacks that killed nearly 3,000.
The appeals court in Virginia rejected his claim his conviction was invalid as the government had failed to provide evidence he could have used in defence.
"Moussaoui challenges the validity of his guilty plea and his sentences" on the various counts, the three-judge panel said in its ruling.
"We affirm Moussaoui's convictions and sentences in their entirety."
The appeals court also brushed aside Moussaoui's lawyers' claims that his guilty plea was invalid.
"The finality of the guilty plea, entered knowingly, intelligently, and with sufficient awareness of the relevant circumstances and likely consequences, stands," the court said in its statement.
During his original trial in 2005, he had testified that his role was to hijack a fifth plane and crash it into the White House.
After the verdict, Moussaoui changed his stance and denied any involvement in the attacks.
But lawyers from the US justice department told the appeal court that Moussaoui had wanted to plead guilty, even though this was against the advice of his lawyers.
At his appeal, his lawyers also argued that his preparations for the trial had been damaged because he was not aware of classified evidence held by the government that might have helped his case.
These claims were also thrown out by the appeals court, which said Moussaoui was both aware of his rights and knew the gist of the classified evidence in question.
Moussaoui is currently serving his life sentence in a super-maximum federal security prison in Colorado.
During his original trial, the jury in Alexandria, Virginia, ruled that Moussaoui's lies to US investigators after he was arrested led directly to at least one death on 9/11.
He was convicted of several counts of conspiracy - including to commit acts of terrorism and destroy aircraft - which carry the death penalty.
Though the prosecution had requested it, he was spared death because of a single vote against the penalty by an anonymous juror.