By Gavin Hewitt BBC correspondent at the courtroom in Alexandria

As soon as Zacarias Moussaoui - dressed in green prison overalls - took the stand, the judge asked him whether he still pleaded guilty to charges relating to the attacks on 11 September.

Mr Moussaoui - speaking with a heavy French accent - said he was guilty of four of the six charges. They included conspiracy to commit acts of terrorism and air piracy, including using planes as weapons of mass destruction.

But Judge Leonie Brinkema wanted to be certain that Mr Moussaoui - who was representing himself - fully understood what he was pleading guilty to.

She painstakingly explained that the charges carried the death penalty and that the conspiracy ended with the deaths of thousands of people on 11 September.

'Enemies'

It was apparent that Mr Moussaoui did not fully understand how conspiracy charges worked.

Several times he said he wanted to explain what part of the charges he was guilty of. The judge told him he couldn't pick and mix.

There are doubts about how strong the accused's links to 11 September are

He said he wanted to tell a jury why he came to the United States, what he did and what he did not do. The 12 Americans on the jury, he said, may be my enemies but "among them I might find an honest enemy".

Mr Moussaoui had no difficulty in accepting that he was a member of al-Qaeda, had been to Kandahar - the former Taleban stronghold in Afghanistan - and might have provided training and a guest house to terrorists. What he denied was involvement in the 11 September attacks.

Mr Moussaoui said he wanted to plead guilty to some details, but he had become aware that there were certain people that "wanted to link me to certain facts that will lead to my death".

He then asked for a 10-minute recess. When he returned, he said he "had an obligation to his creator. I have to withdraw my guilty plea".

The judge said it was "absolutely right", "not an unwise decision". She had already indicated that she was unlikely to accept his guilty plea.

'British agent involved'

The case will now be heard before a jury in the autumn. The US Government will then have to provide evidence linking Mr Moussaoui to the 11 September hijackers, and there are already doubts as to how strong those links are.

It will be a high profile case of the man who the government regards as the "20th hijacker" - 19 men suspected of involvement in the conspiracy died in hijacked airliners that crashed into prestige buildings on 11 September.

Just before the court session ended, Mr Moussaoui said that a British agent was "an important part of the conspiracy ", and he would reveal more when he had his day in court.