The death penalty trial of a confessed al-Qaeda conspirator was unexpectedly halted today by a judge who criticised Government lawyers for violating Zacarias Moussaoui’s constitutional rights by coaching witnesses.

Leonie Brinkema, the federal judge hearing the case, said: "In all the years I’ve been on the bench, I have never seen such an egregious violation of a rule on witnesses."

Her intervention represents a humiliating set-back for the US administration which has spent more than four years trying to bring someone to justice for the September 11 attacks on America.

Moussaoui, a 37-year-old French Moroccan, is the only person to be tried in a US courtroom for direct involvement in the attacks, even though at the time he was already under arrest for immigration violations.

The Justice Department says that he deserves to die through lethal execution because he lied to investigators about what he knew of al-Qaeda’s plans to fly aeroplanes into buildings.

Today’s development came after Carla Martin, a lawyer from the Federal Aviation Administration, admitted coaching at least four witnesses by showing them transcripts of evidence - a deliberate attempt to "shape their testimony", according to defence lawyers.

"This is the second significant error of the government affecting the constitutional rights of this defendant," Judge Brinkema said. "More importantly, it affects the integrity of the criminal justice system in the United States."

Last week, the judge ruled out of order a prosecution question that Moussaoui had a responsibility to tell the FBI about his terrorist links. In remarks which may be significant for the outcome of the trial, she warned the prosecution it was treading on shaky legal ground because there was no precedent for a defendant’s failure to act resulting in the death penalty.

Edward MacMahon, the defence attorney, moved to have the case thrown out - saying "this is not going to be a fair trial" - or at at least have the FAA witnesses removed.

Even David Novak, the prosecutor, acknowledged that witness coaching was "horrendously wrong". But he said the case should be allowed to proceed after rigourous cross-examination of the witnesses. Judge Brinkema said she would need time to study what to do and adjourned the court until Wednesday.

Moussaoui has already pleaded guilty to conspiracy charges in connection with attacks so, even if Judge Brinkema declares a mis-trial, he will still spend the rest of his life in prison.

His court-appointed lawyers, with whom he is not speaking, claim he is mentally unstable and was - at most - only a fringe character in al-Qaeda whose leaders regarded him as unreliable.

They are also attempting to highlight the the Government’s negligence in failing to prevent the attacks despite being given a series of clues in the days leading up to September 11. They will suggest Moussaoui knew less about the plot than many US federal agents.

"You can’t judge him to get revenge for 9/11," or for the failure of the US government to act on other leads, Mr MacMahon told the jury, "no one should be executed on such flimsy evidence, not even a member of Al Qaeda."

But the Government remains eager to use the trial as catharsis for both its own troubles and the anguish of victims’ relatives. It has made special arrangements to accommodate family members in the court and around television screens at six locations elsewhere in the US.

While Moussaoui may not be an ideal proxy for the 19 suicide hijackers, he does at least proudly - and loudly - declare his membership of al-Qaeda, his hatred for the US and his willingness to kill innocent civilians.

Throughout the first week of the trial, Moussaoui sat quietly through proceedings clad in a green prison jumpsuit and a white knitted cap, frequently stroking his thick beard. But at the end of each session in Alexandria, Virginia, he has made a ritual of jumping up and shouting a declaration such as "God curse America," or "God bless bin Laden."

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