A UN-backed special court in The Hague has rejected an appeal against war crimes convictions by lawyers representing former Liberian President Charles Taylor.
It ruled that his convictions had been proved beyond doubt.
Taylor appeared impassive in court as the judge upheld his convictions and 50-year sentence.
He was sentenced in May 2012 for aiding rebels who committed atrocities in Sierra Leone during its civil war.
His lawyers had argued that there were legal errors during his trial.
Taylor, 65, was found to have supplied weapons to the Revolutionary United Front rebels in exchange for a constant flow of so-called blood diamonds.
He was found guilty at his trial of 11 crimes including terrorism, rape, murder and the use of child soldiers by rebel groups in neighbouring Sierra Leone during the vicious civil war of 1991-2002.
Judge Richard Lussick said at his trial that they were "some of the most heinous crimes in human history".
Taylor has always insisted he is innocent and his only contact with the rebels was to urge them to stop fighting.
He became the first former head of state convicted by an international war crimes court since World War II.
"The appeals chamber... affirms the sentence of 50 years in prison and orders that the sentence be imposed immediately," Judge George King told the Special Court for Sierra Leone (SCSL) on Thursday.
'Campaign of terror'
The BBC's Chris Morris in The Hague says the court's decision has been closely watched because the guilty verdict was hailed as a landmark, proving that even people at the highest level of power can be held to account.
In its ruling, the special court said that Mr Taylor's personal conduct had a "significant effect on the commission of crimes in Sierra Leone".
It said that he unleashed a campaign of terror against the Sierra Leonean opposition "using terror as its modus operandi".
"The Appeals Chamber is of the opinion that the sentence imposed by the trial chamber is fair in the light of the totality of the crimes committed," Judge King said.
He said that Taylor's lawyers had "failed to demonstrate any errors in the trial chamber's reasoning".
The BBC's Umaru Fofana in Sierra Leone's capital, Freetown, says the hearing was broadcast live on national television.
Presidential spokesman Unisa Sesay told the BBC: "The end of the Charles Taylor trial presents a final closure to a long and sad episode in our nation's history.
"It's fair and open nature, although torturous, is a practical demonstration of accountability and rule of law in state governance."
Correspondents say that Taylor is now expected immediately to serve his sentence in a foreign jail. The UK has offered to accept him at a British prison - other possible destinations include Sweden or Rwanda.
It is likely to take about a week to organise his transfer from The Hague.
Human rights groups have welcomed the outcome of the appeal.
In a statement Amnesty International said that it sent a clear message to leaders across the world that no-one is immune from justice.
"The conviction of those responsible for crimes committed during Sierra Leone's conflict has brought some measure of justice for the tens of thousands of victims," said Stephanie Barbour, head of Amnesty's Centre for International Justice in The Hague.
"The conviction of Charles Taylor must pave the way for further prosecutions."
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