Zacarias Moussaoui, the only person convicted in the US for his connection to the September 11, 2001 terror attacks, appeared to sign his own death warrant today when he told a jury that he knew of the plot to attack New York’s World Trade Center.

In an extraordinary admission that dismayed his defence lawyers and rescued what had been a disastrous prosecution case, Moussaoui took the stand and within minutes declared: "I had knowledge that the Twin Towers would be hit. I didn’t know the date."

But, crucially for the prosecution, he added: "I knew it would happen after August."

The self-incriminating testimony was a gift for the Bush administration, whose lawyers are seeking to have Moussaoui executed.

During a morning of stunning testimony Moussaoui, who has given different accounts of the events leading up to 9/11, also claimed that he and Richard Reid, the British would-be shoebomber, was supposed to hijack a fifth airplane on September 11, 2001 and fly it into the White House.

The claim about him and Reid was in stark contrast to Moussaoui’s previous statements in which he said he had been training to take part in a separate and later suicide attack on the White House. Reid, subdued by passengers on December 22, 2001 after trying to ignite a shoebomb on a US flight, was sentenced in January 2003 for that plot and has never claimed to be part of the September 11 operation.

Moussaoui, 37, a Frenchman of Moroccan descent, attended flight training school in Minnesota in early 2001. After arousing suspicion in August of that year for trying to learn how to fly a Boeing 747, he was arrested by the FBI and held on immigration charges.

In custody on September 11, 2001, he has confessed that he was training to become a pilot for a later attack on the White House, but has always denied prior knowledge of the attacks on New York and Washington. He pleaded guilty last year to conspiracy charges.

The Administration argues that he should be executed because if he had told investigators the truth after he was arrested, the FBI would have had enough information to stop the September 11 attacks. The jury is being asked to decide whether he should be executed or spend life in prison.

He agreed yesterday with Robert Spencer, a prosecutor, that he lied to investigators after his arrest so that he could allow the September 11 operation to go forward.

"The reason you told lies was so you could allow the operation to go forward," Mr Spencer said.

"That is correct," Moussaoui replied.

He told the court he knew the attacks were coming some time after August 2001 and bought a radio so he could hear them unfold.

"You were looking forward to what happened in that operation," Mr Spencer said.

"Yes, you can say that," Moussaoui responded.

Before he took the stand, his state-appointed lawyers, to whom he does not speak and whom he frequently abuses in court, made a last attempt to stop him from testifying, but failed.

Moussaoui said that he was not part of the September 11 plot. Asked by his lawyer why he signed his guilty plea as"the 20th hijacker" - 19 men were involved in the September 11 attacks - Moussaoui replied: "Because everybody used to refer to me as the 20th hijacker and it was a bit of fun."

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