The family of a British citizen illegally abducted and detained in Ethiopia has appealed to Boris Johnson to step up efforts to secure his release, as they prepare to spend a third Christmas without him.

Andy Tsege, who was granted political asylum in the UK in 1979 and has lived in Britain ever since, was kidnapped and illegally rendered to the east African state in June 2014 at the behest of the Ethiopian government, as part of a widely criticised crackdown on dissidents and civil rights activists.

Speaking from the family home in London two years after the family were last allowed a phone call with Tsege, his daughter Menabe said: “We’re so sad that our dad isn’t with us for Christmas. It’s been two years now since we last heard his voice, and we are so worried about him. I was seven when my dad disappeared, and now I’m nearly 10 – I feel like I’m really growing up without him. I can’t believe the government isn’t doing more to help us.”

A prominent figure in an Ethiopian opposition party, Tsege was kidnapped by Ethiopian agents at Sana’a international airport in Yemen, and imprisoned without access to a fair trial.

Four years earlier he had been condemned to death on terror charges at a trial held in his absence. The trial proceedings were described by US diplomats as “lacking basic elements of due process”. Ethiopia has a reputation as one of the most authoritarian and repressive regimes in Africa.

A letter from Foreign Office minister Tobias Ellwood to the legal charity Reprieve, which is representing the 61-year-old, states: “We are maintaining our focus on seeking to ensure Mr Tsege’s health and welfare, and achieve regular consular access.”

Lawyers say the statement waters down previous claims from the Foreign Office about how much headway had been achieved on the case. Johnson said over the summer that “progress has been made”, and in particular that regular consular access was now in place.

Reprieve said the latest update showed the UK government was retreating from that claim and omitted any reference to the legal representation for Tsege that Johnson’s predecessor, Philip Hammond, said he had secured in June.

The apparently continuing lack of regular consular access means Tsege is unable to freely describe the treatment he has received in jail. Independent experts have raised fears that he has been tortured.

Tsege’s partner, Yemi Hailemariam, said she was despairing at the UK government’s ineffective approach. “It’s heartbreaking to have to prepare my kids for a third Christmas without their dad, and to explain to them why the government isn’t doing more to bring him home. I can’t bear the thought that next year we might be in the same position.”

Harriet McCulloch, deputy director of the death penalty team at Reprieve, said: “It’s nothing short of a disgrace that Andy is spending another Christmas in illegal detention, at the hands of a government that’s subjected him to a series of grave abuses – from a political in absentia death sentence to kidnap, rendition and torture. Enough is enough – Boris Johnson must urgently seek Andy’s return to his partner and kids in the UK.”

Last Tuesday there was cross-party criticism of the handling of cases such as Tsege’s involving Britons who have been detained unjustly abroad. MPs condemned the Foreign Office for declining to intervene strongly in cases involving detained Britons for fear of “interfering” with foreign judicial systems.

Several MPs raised the fact that Ethiopia received significant amounts of UK assistance, including for its security forces. The debate, held amid growing disquiet over a “downgrading” of human rights at the Foreign Office, also confirmed that the Ethiopian government had told MPs that Tsege had no prospect of an appeal.

A Foreign Office spokeswoman said: “The British government has provided significant support to Mr Tsege and his family, and to suggest otherwise is simply incorrect. From the moment we heard about his detention we pressed for consular access and have now been able to visit Mr Tsege a number of times.

“The foreign secretary has made it clear to the Ethiopian government that Mr Tsege must be given access to legal representation, as agreed by Prime Minister Hailemariam [Desalegn Boshe] in June. “And we will press them on this matter until they follow through on their commitment.”