The judge hearing America's only trial of a 9/11 conspirator has banned six witnesses from testifying after discovering that they had been coached.

The ruling by District Judge Leonie Brinkema wipes out half of the prosecution's case against Zacarias Moussaoui, who has admitted conspiracy in al-Qaeda plots to use passenger jets as terror weapons.

Moussaoui was arrested a month before the quadruple hijackings that killed 3,000 people, but US prosecutors are arguing for the death penalty for him, saying that he did not reveal information that could have prevented the attacks.

The witnesses that have been excluded from giving evidence were all due to talk about aviation security, the issue at the heart of the trial. Prosecutors are now wondering whether to appeal.

Relatives of 9/11 victims, who have been watching the proceedings live on television screens set up in courtrooms around America, said that they were appalled by the development.

"I felt like my heart had been ripped out," said Rosemary Dillard, whose husband Eddie was killed aboard the plane that crashed into the Pentagon. "I felt like my husband had been killed again. I felt like the Government had let me down again."

Judge Brinkema's remit is to decide on a sentence for Moussaoui, who was arrested after his instructors at a US flight school became suspicious.

She spent all day in closed court session yesterday, listening to the six witnesses who, it emerged, had been sent extensive transcripts of evidence previously heard in the trial in order to prepare them for their day in court.

The transcripts were sent out by Carla Martin, a lawyer for the Transportation Security Administration, with a covering note explaining that they might help them in answering questions. By well-established legal convention, witnesses are banned from hearing previous evidence in court cases in case it affects their own testimony.

All six said that they had received the e-mails, but did not plan to change their evidence as a result. Ms Martin herself also appeared briefly in court, but did not testify because she could not find a lawyer to represent her. The judge read Ms Martin her rights and warned she might face civil or criminal proceedings for contempt of court.

"I don't think in the annals of criminal law there has ever been a case with this many significant problems," said Judge Brinkema, who considered abandoning the trial altogether but eventually decided to adjourn it until Monday.

"I am removing from this case any and all witnesses and evidence dealing with the aviation component. What we have in this case... is that six witnesses, two for the government and four potential defence witnesses, were tainted."

The missing testimony was expected to deal with how much the Federal Aviation Administration already knew about possible terror threats to airlines prior to 9/11, and what security measures were in place.

The Government is trying to prove that if Moussaoui had not lied to the FBI when he was arrested, the FAA would have been able to thwart the attacks through increased security measures on aeroplanes.

Edward MacMahon, Moussaoui's attorney, said: "This is Mr Moussaoui's trial and it's one that needs to be fair from a constitutional standpoint, and it just flat out isn't."

The judge adjourned the sentencing trial until Monday to give the prosecution time to decide whether to appeal.

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