Radovan Karadzic, the former Bosnian Serb leader convicted of genocide during the conflict in the former Yugoslavia, is to serve the rest of his life sentence in a British jail.
The Foreign Office said he would be transferred to a UK prison from a UN detention unit in the Netherlands.
Karadzic, 75, was also found guilty at his 2016 criminal tribunal of war crimes and crimes against humanity.
His original 40-year sentence was increased at an appeal hearing in 2019.
His conviction for genocide related to his responsibility for the murder of more than 7,000 Muslim men and boys at Srebrenica in 1995.
It was considered one of the worst massacres in Europe since World War Two.
Judges at the International Criminal Tribunal for the former Yugoslavia also held Karadzic responsible for the siege of Sarajevo, a campaign of shelling and sniping which lasted more than three years and led to the deaths of an estimated 10,000 civilians.
Foreign Secretary Dominic Raab said: “Radovan Karadzic is one of the few people to have been found guilty of genocide…
“We should take pride in the fact that, from UK support to secure his arrest, to the prison cell he now faces, Britain has supported the 30-year pursuit of justice for these heinous crimes.”
Mr Raab described Srebrenica as “the darkest moment in European history since the Holocaust”.
“I think we have got a moral duty and I think we have a sense of national purpose in trying to hold to account the perpetrators of the very worst crimes,” Mr Raab said.
“Back since Nuremburg [the German city where Nazi leaders were prosecuted after World War Two] we have played that role.”
He added: “If we want to deter these kind of crimes from happening, if we want to give justice to the many thousands of victims, I think it is right we do our bit.”
The UK was one of the signatories to enforcement agreements with the United Nations for sentences passed by its tribunals related to the former Yugoslavia.
UN officials confirmed they had asked the UK to “enforce the sentence”.
They did not provide a date for the move but said “all necessary measures” should be taken “to facilitate Karadzic’s transfer… as expeditiously as possible”.
The conflict in the former Yugoslavia stemmed from tensions among ethnic groups, following the death of President Tito in 1980.
Calls for more autonomy led in 1991 to declarations of independence in Croatia and Slovenia and fighting with the Serb-dominated Yugoslav army.
Bosnia, with a complex mix of Serbs, Muslims and Croats, was next to try for independence. Bosnia’s Serbs, backed by Serbs elsewhere in Yugoslavia, resisted.
Karadzic, a former psychiatrist, was president of the Bosnian Serb entity Republika Srpska during the Bosnian War in the 1990s.
After the war, he hid for years – masquerading as an expert in alternative medicine – before his eventual arrest in Serbia in 2008.
In November 2017, former Bosnian Serb army commander Ratko Mladic was sentenced to life in prison on similar charges of war crimes and genocide. A ruling in an appeal against his conviction is to be announced next month in The Hague.
