The fiance of Aras Amiri, a British Council employee imprisoned in Iran on spying charges, has accused the UK government of being “utterly blind to their responsibility” to try to secure her release.

An appeal against Amiri’s 10-year sentence was rejected without a hearing earlier this week, in a decision that prompted her family to call on the UK government to intervene.

Now her fiance, James Tyson, has revealed that British officials initially refused to meet to discuss the case and claimed they could not help Amiri because she was an Iranian citizen. In an article for the Guardian, he added: “It is a manifestation of Iran’s own frustrations, but also of the UK’s diplomatic failings.”

Hitting out at the “depravity” of diplomatic protocol, he accused the Iranian government of imprisoning her for the sake of diplomatic advantage, “shooting itself in the foot to spite an enemy”.

And in the strongly worded article he said British officials were “offering platitudes while remaining utterly blind to their own responsibility”.

He blamed both the Iranian and UK governments for failing to enter into dialogue about the case. He said: “She’s caught in the middle of two global powers who refuse, or don’t know how, to talk to each other.”

Amiri, 33, who worked as an artistic affairs officer for the British Council, was arrested last year when she was visiting her ailing grandmother in Iran. In May she was sentenced to 10 years in prison on spying charges after she claimed she refused to become an informant for Iran’s intelligence service.

Tyson, who also works for the British Council, said the Iranian authorities know the charges against her are bogus. He pointed out that Iran helped Amiri organise events in the UK aimed at showing off the best of Iranian art in the UK.

He said: “For that same government to now use Aras as a political pawn against the UK with a maximum prison sentence is a rather hellish sign of the lengths power will go to, and how far people will go ‘just following orders’.”

He went on to point to the diplomatic protocol governing the handling of the case, saying: “This stand-off of government hostage-taking reveals the depravity beneath the ‘respectable’ codes of modern international relations.”

In an interview with the BBC’s Victoria Derbyshire show on Wednesday, Tyson said the Foreign Office had been reluctant to take up Amiri’s case despite her status as a UK resident and employee of the British Council.

He said: “In many ways, the British government is responsible for her wellbeing and [has a] duty of care, particularly because that’s the reason why she was being held. Initially they would not meet with us. When I tried to meet with the Foreign Office we were simply told that it’s actually best for her family to deal with the situation.

“If a UK diplomat had been arrested in another country, and you were told your mother should sort out the problem, it’s not really necessarily very helpful.”

He conceded that the Foreign Office had since changed its stance and is now “very concerned”. But he said it was still refusing to enter into talks about the case. Asked what he would say to the foreign secretary, Dominic Raab, Tyson said: “I would say, ‘Get on the phone and talk to the Iranian government and say this can’t happen, you know, we need to find a way of solving this.’

“I think that involves listening on both sides. And being quite adult about these things. It is embarrassing both the Iranian government and the British government.”

A Foreign Office spokeswoman insisted that UK diplomats were trying to secure Amiri’s release. The UK ambassador in Tehran, Rob Macaire, raised the issue with Iran’s foreign ministry in a meeting on Wednesday, she said. Earlier this year the former foreign secretary, Jeremy Hunt, had a meeting with Tyson about his fiancee’s plight.

Tyson also revealed that Amiri had become close with her cellmate, Nazanin Zaghari-Ratcliffe, the Iranian-British citizen serving a five-year sentence for alleged spying.

He said: “They’re sort of housemates in many ways. They’re very supportive of each other … telling stories, sharing books they have, and cooking together. And that’s how they try and they pass their days. In many ways it’s good that they could have each other, actually, through this time.”

Tyson said that he was planning to marry Amiri in Iran last year when she was briefly released on bail. He had been granted a visa but was warned he was likely to be arrested or deported if he tried to enter Iran.