In February Abdul Kader Mullah, of the Jamaat-e-Islami party, was convicted by a special tribunal of crimes during the 1971 war of independence with Pakistan.
The life sentence imposed at the time sparked angry street protests.
Abdul Kader Mullah’s supporters said the charges were politically motivated but his opponents said that sentence was too lenient.
Thousands staged protests around the country demanding the death penalty for Abdul Kader Mullah, who had been convicted of charges which included overseeing massacres during the bloody struggle for independence.
The protests prompted parliament to amend a law which allowed the state to appeal against any verdict reached by the war crimes tribunal.
This meant that alongside Abdul Kader Mullah’s appeal against his verdict the Supreme Court also heard an appeal from the government which wanted a tougher sentence handed down.
Prosecutor Ziad Al Malum told AFP news agency that the decision to increase the sentence was approved by four to one at the court.
Defense lawyer Tajul Islam told the agency he was “stunned” by the verdict.
“This is the first time in South Asian judicial history that a trial court sentence has been enhanced by a Supreme Court,” he said.
Local media reports cite Abdul Kader Mullah’s lawyers saying they will petition for this harsher sentence to be reviewed, but the attorney general has said there can be no appeal against a Supreme Court verdict.
Correspondents say the only option left for Abdul Kader Mullah would be to seek a presidential pardon.
Abdul Kader Mullah’s conviction and subsequent sentences handed down to other Islamist leaders by the tribunal over the last few months have unleashed a wave of unrest, pitting supporters of Jamaat, who accuse the government of pursuing a political vendetta, against pro-government groups.
More than 100 people have been killed since January in the violence.
This is just the first of a series of appeals against convictions handed down by the tribunal. Several other high-profile Islamist leaders have been sentenced by it including 90-year-old Ghulam Azam, the former leader of Jamaat-e-Islami, who was jailed for 90 years in July.
Abdul Kader Mullah, 65, was the assistant secretary-general of Jamaat and the war crimes tribunal found him guilty of five out of six charges, including murder.
He was accused of being behind a series of killings including large-scale massacres in the Mirpur area of Dhaka, which earned him the nickname of “koshai” or butcher of Mirpur, and made him one of the more feared Jamaat leaders.
The special court was set up in 2010 by the current Bangladeshi government to deal with those accused of collaborating with Pakistani forces who attempted to stop East Pakistan (as Bangladesh was then) from becoming an independent country.
But human rights groups have said the tribunal falls short of international standards.
Bangladesh government figures estimate more than three million people were killed during the independence war. Other researchers put the figure at between 300,000 and 500,000.
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