The trial of Saddam Hussein was so flawed that its verdict is unsound, the advocacy group Human Rights Watch says.
Court proceedings were marked by frequent outbursts
HRW said "serious administrative, procedural and substantive legal defects" meant the 5 November trial for crimes against humanity was not fair.
The Iraqi government has dismissed the report, telling the BBC that the trial was both "just and fair".
The ex-Iraqi leader has two weeks to lodge an appeal but his lawyer claims he has been blocked from doing so.
Chief defence lawyer Khalil al-Dulaimi told the BBC his team had been prevented from filing appeal papers. Under Iraqi law it must be done within a month of sentencing.
However, the chief prosecutor, Jafaar al-Mousawi, has told the BBC it was a fair trial.
He said the appeal would be automatic because a death sentence had been passed - and that the relevant papers had been sent to the appeal court.
Mr al-Mousawi's claims that the trial was fair were echoed by Iraqi Foreign Minister Hoshyar Zebari.
"We believe strongly that the trial was fair and Saddam Hussein had every right to defend himself," he told the BBC.
"The procedure, I think, in the court, was witnessed by the whole world."
"It wasn't something done behind closed doors or through summary justice, as was the case during his rule. And we stand really by the court's verdict, and we believe the trial was just and fair."
Saddam Hussein and seven co-defendants all faced charges of crimes against humanity relating to the deaths of 148 people in the mainly Shia town of Dujail following an assassination attempt on the Iraqi leader in 1982.
Two of his co-defendants also received death sentences.
Judge Raouf Abdel Rahman replaced an earlier chief judge
Saddam Hussein is now being tried on a different set of charges relating to a military campaign against ethnic Kurds in the late 1980s, in which more than 180,000 people are alleged to have died.
The New York-based HRW group said the trials were among the most important since the Nazi trials in Nuremberg after World War II.
They "represent the first opportunity to create a historical record concerning some of the worst cases of human rights violations, and to begin the process of a methodical accounting of the policies and decisions that give rise to these events", the report said.
The BBC's David Loyn in Baghdad says HRW's disappointment is the greater because it considers the Iraqi court to have failed to take account of the international significance of this trial and the one currently under way.
Supporters of war crimes trials say that they can improve healing after conflict, our correspondent says, but like so much else in post-Saddam Iraq, his trial does not seem to have achieved that lofty ambition.
HRW based its scathing assessment on extensive observation of court proceedings, and interviews with judges, prosecutors, defence lawyers and court administrators involved.
The trial took just over one year to complete and was the first case brought before the Iraqi High Tribunal.
Proceedings were marked by frequent outbursts by both judges and defendants.
Three defence lawyers were murdered, three judges left the five-member panel and the original chief judge was replaced.
Defence lawyers boycotted proceedings but HRW said court-appointed counsel that took their place lacked adequate training in international law.
In addition, important documents were not given to defence lawyers in advance, no written transcript was kept and paperwork was lost, said HRW.
The defence was also prevented from cross-examining witnesses and the judges made asides that pre-judged Saddam Hussein.
The US-led Coalition Provisional Authority decided that the Dujail trial would be held by an Iraqi court in Iraq, ruling out an international tribunal or a mixed Iraqi-international court under UN auspices, the HRW report said.
Because Iraqi lawyers and judges had been isolated from international criminal law, this decision resulted in a court that lacked the expertise to prosecute crimes against humanity on its own, the report said.
Three defence lawyers were murdered during the trial
Defence counsel come under criticism in the report for trying to use the court as a political platform.
Meanwhile, the Iraqi government was guilty of influencing the independence of the judges, the report said, to the extent that the first chief judge resigned.
"Under such circumstances the soundness of the verdict is questionable," HRW concludes.
"In addition, the imposition of the death penalty - an inherently cruel and inhumane punishment - in the wake of an unfair trial is indefensible."