A British-Iranian woman sentenced to five years in jail in Iran on unspecific charges relating to national security has had her conviction upheld in the appeals court, according to judiciary officials.

Nazanin Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, the news agency’s charitable arm, appealed against her sentence earlier this month in what was her last legal opportunity to challenge it.

“Her sentence has been confirmed,” Gholam-Hossein Mohseni-Ejei, the judiciary spokesperson, told reporters in Tehran on Sunday, the semi-official Isna news agency reported.

Zaghari-Ratcliffe, 38, is serving her prison term in the women’s ward of Tehran’s notorious Evin prison alongside other Iranians behind bars on political or religious grounds. She was originally found guilty in September.

The Iranian Revolutionary Guards, the elite forces that arrested her in April at the airport while she was about to return to the UK after a family visit, have accused her of orchestrating a “soft overthrow” of the Islamic Republic – accusations her husband has vehemently denied.

Her imprisonment, which is part of a string of cases involving dual nationals, has cast a shadow on the thawing in Tehran-London relations in the wake of the landmark nuclear deal. The Iranian judiciary has also arrested a number of other Iranians with dual citizenship from western countries in recent years.

Tobias Ellwood, the Foreign Office minister for the Middle East, visited Tehran earlier in the week, meeting Zaghari-Ratcliffe’s family at the British embassy in Tehran. It was not clear if Ellwood had discussed her imprisonment with his Iranian counterparts.

Last year, the British ambassador also met Zaghari-Ratcliffe’s two-year-old daughter, Gabriella, who has been placed in the care of her Iranian grandparents. Gabriella was accompanying her mother during the visit to Iran.

After her arrest, Zaghari-Ratcliffe was transferred to the southern Iranian city of Kerman, where she was held for weeks in an unknown location before being taken to Evin prison.

Although the exact reasons for her incarceration are unclear, Iranian authorities have hinted that her arrest is linked to the 2014 imprisonment of several employees of an Iranian technology news website, according to Amnesty International. They were given long prison terms for participating in a BBC journalism training course. Zaghari-Ratcliffe was a project assistant at the BBC’s Media Action, the broadcaster’s international development charity, in 2008-09. The BBC is loathed by the Iranian establishment, mainly for its Persian service, which is watched by millions of Iranians via illegal satellite dishes.

Richard Ratcliffe has claimed his wife is being used as a pawn in political deals with the UK. Iranian officials have denied this, but Tehran has shown a pattern of behaviour in previous cases involving dual nationals. Last year, it notably released the Washington Post journalist Jason Rezaian alongside other Iranian-Americans, swapping them for Iranian nationals held in US jails for crimes including violating economic sanctions.

Zaghari-Ratcliffe’s arrest and sentencing has been carried out by institutions that act independently of President Hassan Rouhani’s government. Rouhani has been particularly at odds with the judiciary and earlier this month engaged in a public spat with its chief over financial transparency of the justice system.

Ratcliffe wrote to Rouhani this month, complaining about his wife’s prolonged imprisonment and saying that despite the trial no crime has been identified in public.

“I am also the father of Gabriella Ratcliffe, two years old, who has had her British passport confiscated by the Revolutionary Guard, and not returned despite family and government requests. Nazanin has occasional visits from Gabriella via a blindfold, at the discretion of her captors, far less than the visits reported to the UN. I am reduced to parenting by Skype,” he wrote.

He added: “I have stated publicly I regard both my wife and daughter as being held by Iranian factions for other agendas, as bargaining chips for international and domestic politics, inventing lies to protect their economic interests. This is a politics that it is unfair on my family to be involved in. It is a politics for which my family has paid a heavy enough price.

“I have now not seen my daughter or wife for 10 months. This is a very long time not to be able to see how they are.”

Richard Ratcliffe said on Sunday: “The lack of justice in Nazanin’s case continues to be a stain on Iran. The continued attempt to frame Nazanin behind secrets and lies brings shame.

“It is a needless waste of a mother and child’s life for their own political bargains and economic interests [...] And yet this ordeal continues, with all its consequences for Nazanin and Gabriella, and for all of us watching on. It is no way to toy with people’s lives. There is no way we will let it rest. Nazanin will not be forgotten.”

More than 850,000 people have signed a petition on change.org, calling on Iran to release her.

An FCO spokesman said: “We are aware of reports that Mrs Zaghari-Ratcliffe’s appeal has been decided and are urgently seeking clarification.”