In a lengthy summary, read out in court, the judges ruled that Mr. Taylor's sentencing had been "fair and reasonable," rejecting the defense appeal for his immediate release and the prosecution request that his jail term be extended to 80 years.

Mr. Taylor was found guilty in April 2012 on all counts of an 11-count indictment alleging war crimes and crimes against humanity relating to his role in aiding murderous rebels who committed atrocities in Sierra Leone during its civil war in the 1990s.

He was accused of fomenting widespread brutality that included murder, rape, the use of child soldiers, the mutilation of thousands of civilians and the mining of diamonds to pay for guns and ammunition.

In May 2012 he was sentenced to 50 years in prison — the first former head of state convicted by an international tribunal since the Nuremberg trials in Germany after World War II.

After the reaffirmation of his conviction on Thursday, the hunt for Mr. Taylor's suspected fortune could well resume.

Under the tribunal's rules, victims in Sierra Leone, particularly thousands who suffered during the attack in 1999 on Freetown, the Sierra Leone capital, are entitled to seek reparations in national courts.

Experts believe these civil cases could go on for years because Mr. Taylor's widely rumored assets have proved elusive.

Investigators have succeeded in freezing $8 million held by his relatives and associates.

But, according to court filings, the tribunal failed to discover the final destination of millions traced through Liberian and other banks while he was in power and the court's investigators were unable to prove his presumed ownership in a number of companies.

With Mr. Taylor claiming he was "partially indigent," the nations that helped fund the tribunal — the United States was the largest donor — have had to cover his legal bills and the broader expenses of a trial that cost more than $20 million.

The ruling is expected to have an impact well beyond the Taylor trial and is likely to add fuel to the present controversy over the definition of "aiding and abetting" human rights crimes as it applies to senior officials who are far away from the battlefield or scenes of a crime. In February, appeals judges at the Yugoslavia tribunal were widely criticised and accused of re-writing customary international law when they unexpectedly overturned the conviction of the former chief of the Yugoslav Army, Gen. Momcilo Perisic, who like Mr. Taylor, was far removed from any crime scenes.

The judges ruled that although many atrocities had taken place, there was no evidence that Gen. Perisic had specifically directed any crimes and ordered him released. But at the Special Court for Sierra Leone, whose statute says that appeals judges "shall be guided" by appeals decisions of the United Nations tribunals for Rwanda and Yugoslavia, the panel went against the Perisic decision.

The Taylor panel ruled that it was not persuaded by the conclusions of the Perisic case because the judges had not provided a clear and detailed analysis of why a commander had to give explicit orders, to be held accountable for atrocity crimes under international law. Mr. Taylor knew the strategy of the groups he supported and of their intention to commit crimes, but nonetheless provided substantial aid.

The defense had appealed the verdict and sentence on 42 grounds, arguing that the Special Court on Sierra Leone had made "systematic errors in the evaluation of evidence and in the application of law" sufficiently serious to "reverse all findings of guilt entered against him," the United Nations-backed tribunal said in a statement summing up the case. Defense lawyers also argued that the sentence was "manifestly unreasonable."

For its part, the prosecution said the sentence was not "reflective of the inherent gravity of the totality of his criminal conduct and overall culpability" and should be increased to 80 years.

But after a detailed review of their findings, read out to the court, the panel upheld the bulk of the conclusions drawn by judges at his trial which lasted from 2007 to 2011. Dressed in a dark suit, white shirt and pale gold tie, Mr. Taylor sat impassively through the 90-minute hearing that confirmed his sentence.

After discussions earlier in the trial, Mr. Taylor, 65, had been expected to serve his sentence in a British maximum security prison, but he is now seeking to be transferred to a jail in Rwanda. Given his age, he will likely spend the rest of his life in prison.

During weeks of testimony, Mr. Taylor said he had heard about atrocities in Sierra Leone but that he would "never, ever" have permitted them.

But the presiding judge at the appeals hearing on Thursday, George Gelaga King, who is from Sierra Leone, said Mr. Taylor had been fully aware of the crimes being committed by rebel groups he advised and encouraged.

"Their primary purpose was to spread terror. Brutal violence was purposely unleashed against civilians with the purpose of making them afraid, afraid that there would be more violence if they continued to resist," Judge King said.

Marlise Simons reported from Paris, and Alan Cowell from London.