Zacarias Moussaoui has pleaded guilty in a US court to helping al Qaida carry out the September 11 terrorist attacks.
The 36-year-old is the first person to be convicted in the US in connection with the hijackings and now faces the death penalty.
In a brief but highly-charged court appearance in Alexandria, Virginia, Judge Leonie Brinkema accepted guilty pleas to six counts of conspiracy in connection with the 2001 attacks that killed almost 3,000 people.
Moussaoui, a French citizen who once lived in London, stood calmly before the judge wearing a green prison-issue jumpsuit and sporting a full beard.
"I expect no leniency," he said, after signing the documents.
He came to the US to receive flight training and had known about al Qaida's plans. Part of his mission had been to fly a jet into the White House if the US did not co-operate in freeing sheikh Omar Abdel Rahman, sentenced to life in prison for plotting to blow up several US targets.
He denied any direct role in the September 11 attacks but said he was part of a "broad conspiracy" to use planes as weapons.
The papers revealed that Osama bin Laden had personally selected him to take part in the operation.
US Attorney General Alberto Gonzales confirmed the government would be seeking the death penalty, despite his guilty plea. "The fact that Moussaoui participated in this terrorist conspiracy is no longer in doubt," he said.
"Moussaoui and his co-conspirators were responsible for the deaths of thousands of innocents on September 11, each one a son or daughter, father or mother, husband or wife."
Mousaoui was arrested in Minnesota in August 2001 after raising suspicions at a flight school. He had wanted to plead guilty three years ago but changed his mind, claiming he was not involved in the hijackings.
His lawyers, who advised against entering a guilty plea, had implored Judge Brinkema to reject the admission, but she found he was "fully competent to plead guilty to the indictment".
Of the six counts - conspiracy to commit acts of terrorism, commit aircraft piracy, destroy aircraft, use weapons of mass destruction, kill US citizens and destroy property - the first four carry the death penalty. Moussaoui has indicated he will fight "every inch" against a death sentence. A date for the hearing has not yet been set.