Andargachew Tsige was found guilty of “terrorism” and sentenced in absentia in 2009 over his role in the opposition group Ginbot 7. He was the organisation’s secretary-general.
The father of three was arrested during a stopover at a Yemen airport in June 2014 and taken to Ethiopia.
Berhanu Tsegaye, Ethiopia’s attorney general, said on Saturday that Andargachew was pardoned “under special circumstances” along with 575 other inmates.
The decisions were made with the “intention of widening political space,” the attorney general told reporters in the capital, Addis Ababa.
Andargachew is expected to be released within the next two days.
Yemi Hailemariam, Andargachew’s wife, said she hoped he would be allowed to return to Britain soon.
“I am so thankful that the pain and anguish my children have had to go through could now soon be coming to an end,” she said in a statement issued by Reprieve, a human rights group.
The pardons are part of reforms that the government has pledged to undertake after violent unrest broke out three years ago, sparked by an urban development plan for Addis Ababa that critics said would trigger land seizures in the surrounding Oromia region.
The protests broadened into rallies over political rights, leading eventually to the resignation of Prime Minister Hailemariam Desalegn in February.
He has since been replaced by Abiy Ahmed, a former lieutenant-colonel in the army and head of Ethiopia’s science and technology ministry.
Tigistu Awelu, chairman of the opposition Andinet party, welcomed the government’s decision to free Andargachew as “hugely significant for bringing about national reconciliation and healing”.
Last week, the government and an exiled opposition party from Oromia opened talks with the aim of enabling it to return to the political fold.
The Oromo Democratic Front (ODF) was formed in 2013 by former members of the Oromo Liberation Front and seeks self-determination for ethnic Oromos, Ethiopia’s largest ethnic group. Its leaders have been living in exile.
AlJazeera