"They executed him twice," said Mark Elliot, director of Floridians for Alternatives to the Death Penalty. "He was still conscious when they put in the chemicals that burn the internal organs. It's exactly like being burned at the stake from the inside."

Diaz, 55, had fought a lengthy legal battle with the state, arguing that death by lethal injection constituted a cruel and unusual punishment.

His final appeal to the US supreme court was rejected an hour before his execution.

His death comes at a time of national debate about the use of injections. A similar case is before the courts in California, where convicted murderer Michael Morales is arguing the method is inhumane. The three-drug cocktail of sodium thiopental, an anaesthetic; pancuronium bromide, which paralyses the body; and potassium chloride, which causes cardiac arrest, is used in 37 US states.

Gretl Plessinger, a spokeswoman for Florida's Department of Corrections, said that Diaz was snoring soon after the first injection, did not regain consciousness and felt no pain throughout. She said a second dose was given because he had liver disease, which slowed his metabolism, but acknowledged that he had taken an unusually long time to die.

The last two executions at Starke were completed within 15 minutes. A statement from Florida governor Jeb Bush's office said: "A pre-existing medical condition of the inmate was the reason tonight's procedure took longer than recent procedures carried out this year."

Mr Elliot said Diaz's death would increase the scrutiny of lethal injections. "His execution, and the way it was mishandled, could lead to a change in the way executions are carried out nationwide," he said.

"If you hang somebody and the rope breaks and you hang them again, you've succeeded in executing them, but you've still botched the original execution."

The execution was the 21st of Mr Bush's eight-year tenure as Florida governor and the last before he leaves office on January 2. His brother George Bush, the US president, oversaw 152 executions in his six years as governor of Texas.

Florida lawmakers voted to switch to lethal injection in 2000 after a series of bungled executions using the electric chair. In the most notorious incident, in 1997, flames shot from the head of a prisoner during an execution.

But Mark Elliot of the group Floridians for Alternatives to the Death Penalty, said the execution showed that the prisoner had been conscious after the administration of a supposedly lethal dose.

"Witnesses said he was moving and trying to speak," he said. "This shows that Florida has no business being in the execution business. Our system is broken from top to bottom."

In a statement, Florida governor Jeb Bush said the state had followed correct procedures. He added: "A pre-existing medical condition of the inmate was the reason the procedure took longer than recent procedures carried out this year."

Diaz's cousin told Associated Press that his family had not known that he suffered from liver disease.

In his final appeal the US Supreme Court Diaz challenged the execution chemicals saying they constituted a "cruel and unusual punishment". His appeal was rejected an hour before the execution began.

Moments before his execution, Diaz again denied the murder. There were no eyewitnesses but Diaz's girlfriend, had told police he had been involved in the killing.

The governor of Diaz's native Puerto Rico, which abolished capital punishment in 1929, had sought clemency for him.

But yesterday, amid intense interest from the media, lawyers and historians, the court of revision refused to posthumously clear Seznec's name. Last year the state prosecutor concluded that fresh evidence cast doubt on the guilty verdict. But judges insisted there were no new facts leading them to doubt Seznec's conviction.

"It is shameful what you are doing!" shouted Seznec's grandson, Denis, 59, from the gallery, accusing the courts of wasting a chance to prove the justice system was able to admit its mistakes.

The case comes as the legal system is reeling from the failures of a case in Outreau in northern France where several innocent people were jailed for years for paedophile offences, with some having their children taken into care. Some of those finally acquitted in that case were present in court with the Seznec family yesterday.

In May 1923 Seznec left Brittany for a trip to Paris with a friend, Pierre Quemeneur, a woodcutter and local official. They were to negotiate with Boudjema Gherdi the sale of 100 Cadillac cars, left by US troops after the first world war. But their car broke down several times and Quemeneur decided to take the train alone. He never arrived in Paris and his body was never found. The following month his suitcase was found at Le Havre with a note in which Quemeneur promised to sell his land to Seznec. Seznec's family argue the document was faked.

At Seznec's trial lawyers claimed that Gherdi did not exist, and that Seznec had planned Quemeneur's murder to take over his land. But Gherdi was later proved to exist. He was a car trader and an informer for a police officer on the case, inspector Pierre Bonny.

The Seznec family questioned whether the police had framed Seznec to cover up a racket in American cars. In 1993 an 85-year-old former shopkeeper retracted statements implicating Seznec, saying she had been coerced by the police.

Bonny joined the French Gestapo during the occupation and was shot by firing squad in January 1945. On the eve of his execution, his son Jacques said he confessed to having sent an innocent man to a penal colony.