Zacarias Moussaoui, the only person in the US charged over the 11 September 2001 terror attacks, is expected to enter a guilty plea in court on Friday.
Zacarias Moussaoui revoked a previous guilty plea
Mr Moussaoui faces a number of charges relating to the attacks, and could face the death penalty if convicted.
It is not clear to which charges he will plead guilty.
Judge Leonie Brinkema pronounced Mr Moussaoui competent to enter the plea after meeting him on Wednesday, despite previous doubts about his mental state.
Mr Moussaoui - born in France to Moroccan parents - tried to plead guilty in 2002 but retracted the plea a week later.
He has also on occasion insulted the judge, lawyers and prosecutors.
"He can be somewhat moody," according to John Clark, the US marshal for the Eastern District of Virginia.
The US government says he was a full member of the original team involved in the 2001 attacks when hijackers took over four passenger planes and deliberately crashed them.
The trial has been beset by delays.
In March, the US Supreme Court refused to examine an appeal by his team for access to US al-Qaeda suspects on security grounds.
Last year, federal judges barred Mr Moussaoui from calling people held at the US prison camp in Guantanamo Bay, Cuba, as witnesses.
The court said the accused could only use government-prepared summaries of the interrogation statements from al-Qaeda suspects in Guantanamo.
The three judges also overruled a lower court decision by deciding that the death penalty could be used if there was a guilty verdict.