The UK government needs to raise the profile of British-Iranian dual nationals held in Tehran’s jails by following the US example and appointing a special envoy for hostages, the wife of one of those held by Iran has said.

In a letter to the UK foreign secretary, Dominic Raab, Sherry Izadi – the wife of Anousheh Ashoori, an Iranian-British dual national sentenced to 12 years – says British policy has been an abject failure, adding that a cross-Whitehall figure dedicated to the issue would be more effective than the prime minister, Boris Johnson, joking with the Iranian president, Hassan Rouhani, as if he is a long-lost friend.

Izadi’s proposal for an envoy, sent on World Human Rights Day, comes after the US and Iran completed a prisoner swap at the weekend that included the release of the US scholar Xiyue Wang after three years in Evin prison, Tehran. The Iranian foreign minister, Javad Zarif, is now proposing all Americans in jail be swapped with the US.

Izadi points out that more British-Iranian dual nationals are imprisoned now than at any other time, and questions whether Raab is simply making empty promises to the relatives of political prisoners. Her husband, to whom she has been married for 35 years, was sentenced to 12 years on espionage charges in August 2019, two years after his arrest. He is also in Evin prison.

In her letter she points out that even though US-Iranian relations “are at an all time low”, the US appointed special envoy for hostages was having more success releasing its political prisoners than the UK.

Donald Trump appointed Robert O’Brien as special presidential envoy for hostage affairs in 2018 – he is now national security adviser – and he is widely regarded to have been influential in negotiating Wang’s release.

In her letter Izadi asks: “How is it that the UK, whose leader was seen chatting away and laughing with President Hassan Rouhani, like long-lost friends who have at last been reunited, has not had even a smidgen of success in securing anyone’s release?”

She explains: “It is clear that the involvement of different Whitehall departments, all with their own agendas, has resulted in an incoherent approach and an abject failure in terms of practical outcome.”

In the case of British dual nationals in Tehran, the Foreign Office, the Ministry of Defence, the Cabinet Office and the Treasury all have an interest. It is widely thought a key motive for the arrest of UK citizens is the Iranian demand for the repayment of about £400m owed by Britain arising from an arms deal in the 1970s.

Izadi writes that: “The British government’s behaviour in failing to recognise the arbitrary detention of its citizens as a form of hostage-taking only serves to undermine the real motive behind these acts; namely, Iran’s desire to use these innocent individuals to gain concessions that are otherwise denied to it. A brief look at the history of the Islamic revolution will make it abundantly clear that the policy of hostage-taking has been pursued by the Iranian regime since the beginning; sometimes for the purpose of prisoner exchanges and at other times for money.”

Her proposal for a UK special envoy has the backing of other relatives of Iranian prisoners such as Richard Ratcliffe, the husband of Nazanin Zaghari-Ratcliffe, who was jailed in 2016.

“You told us, Mr Raab, that ‘there must be good faith on all sides’ for this problem to be resolved”, Izadi continues in her letter. “What better sign of good faith than honouring your country’s debt and putting an end to 40 years of legal wrangling and obfuscation? Iran’s strategy of fulfilling its objectives is wrong and despicable but is the UK’s refusal to honour its legal commitment and debt any less reprehensible?”

Although families of Iranian political prisoners have regular meetings with Foreign Office staff, many families believe there is a lack of political grip and that the presence of a single non-political figure might help end the impasse and raise the profile of the issue.

The UK, and many other states, do not recognise state hostage-taking but the US special envoy attempts to get round this by having a mandate that includes illegal and arbitrary detentions. There are proposals to give the envoy powers to impose sanctions on those involved in hostage taking.