Britain said Thursday its relationship with Ethiopia was being put at risk by the country's year-long detention of opposition leader Andargachew Tsege, an Ethiopian-British dual national.
Addis Ababa said last week that Tsege, 60, who is on death row, would not be pardoned but might escape execution, after reports he was ailing in solitary confinement.
Britain's Foreign Secretary Philip Hammond discussed the case Wednesday with his counterpart Tedros Adhanom Ghebreyesus.
"I am deeply concerned that, a year after he was first detained, British national Andargachew Tsege remains in solitary confinement in Ethiopia without a legal process to challenge his detention," Hammond said in a statement on the phone call.
"I am also concerned for his welfare and disappointed that our repeated requests for regular consular access have not been granted, despite promises made.
"Ethiopia's failure to grant our repeated and basic requests is not acceptable," Hammond added.
"The lack of progress risks undermining the UK's much-valued bilateral relationship with Ethiopia."
Britain is one of Ethiopia's largest donors, with development aid totalling some 303 million pounds ($475 million, 425 million euros) for the period 2015-2016.
Tsege, the leader of the outlawed Ginbot 7 group, who was granted political asylum in Britain, was convicted in absentia in 2009 for terrorism, and was imprisoned a year ago after being controversially extradited from Yemen.
Addis Ababa says he tried to overthrow the Ethiopian government, plant bombs and had organised unrest, as well as training opposition groups in neighbouring long-term foe Eritrea.
Rights groups have criticised Ethiopia's anti-terrorism law, accusing the government of using it to silence dissent and curb freedom of expression.
The Ginbot 7 group is considered a terrorist organisation under Ethiopian law. Its name commemorates the Ethiopian calendar date -- May 15 in the Gregorian calendar -- when post-election violence in 2005 left more than 200 people dead.
Ethiopia's ruling party and its allies achieved a clean sweep in last month's general election, winning all 546 parliamentary seats, the final results showed Monday.
The Ethiopian People's Revolutionary Democratic Front of Prime Minister Hailemariam Desalegn stripped the opposition of the one seat it had held.
Meanwhile Hammond has ordered a review into how Britain's foreign aid is used, following a report that money was being spent on projects including a television game show.
More than £13,000 was budgeted for the question and answer show, due to be aired later this year.
A Foreign Office report said the show aims to engage young people "on UK values of human rights and good governance" according to The Sun newspaper.