Japanese government aid workers who worked on a $22.5m medical assistance package to help Iran fight coronavirus are pressing Tehran to make the final steps to release Nazanin Zaghari-Ratcliffe.

The British-Iranian dual national, who was sentenced to five years’ imprisonment in September 2016, has been on furlough on a tag for three weeks in Tehran, waiting to hear whether she will be allowed to return to her family in the UK.

The Iranian prosecutor’s office is due to update the family on Wednesday on whether she has qualified for clemency under the terms set out by the supreme leader, Ayatollah Ali Khamenei, but such deadlines have slipped in the past, reflecting tensions within the Iranian government.

Zaghari-Ratcliffe’s husband, Richard Ratcliffe, has written to the Iranian ambassador to the UK, saying his wife is in purgatory. Photograph: Dominic Lipinski/PA

Her husband, Richard Ratcliffe, has written to the Iranian ambassador to the UK, Hamid Baeidinejad, thanking him for his role in securing his wife’s partial release, and saying she is in a form of purgatory “with two bags packed, one for a sudden return to the UK and the other for her return to Evin prison”.

The lobbying from members of the Japanese Disaster Emergency Relief and Humanitarian Medical Assistance rescue teams has come in the form of a letter to the Iranian ambassador to Japan, reminding the Iranians that Tokyo donated $22.5m to the World Health Organization specifically to help Tehran fight coronavirus.

The disaster team, based in a number of Japanese hospitals, has been advising Japanese and Iranian officials on the purchase of the specialist medical equipment.

Zaghari-Ratcliffe previously acted as a translator for Japanese disaster relief workers inside Iran in 2003 and 2004, following the Bam earthquake that killed 26,271 people and injured 30,000.

In their letter, the 10 Japanese humanitarian workers write: “Nazanin’s gentle and warm personality and devotion for the victims of Bam created a fantastic teamwork between Japanese team members and Iranians. We all loved her and the colour of our friendship has not faded at all since then.

“Because Nazanin opened the door for us to Iran, we love Iran. We feel emotionally attached to Iran, and we are so much concerned with whatever happened in Iran during the past 16 years. The news of Nazanin’s detention was the worst news for us. Her arrest broke our hearts.”

The specialist aid workers point out that prior to the official government aid announcement they had been privately doing their best to purchase the necessary medical equipment for Iran via their network of professional and medical equipment companies.

“It is purely because we love all the Iranians we met during our mission, including Nazanin. We promise that we will continue to support Iran and Iranians in any way possible at any time. We are looking forward to seeing Nazanin reunited with her family and loved ones”.

Japan’s prime minister, Shinzo Abe, has acted as an intermediary between Iran’s president, Hassan Rouhani, and the west over the breakdown of the nuclear deal, visiting Tehran last autumn.

Richard Ratcliffe strikes a conciliatory note in his letter to the ambassador, writing: “Coronavirus is a reminder to all of us of the commonalities beyond politics, that there are challenges that unite us all. It clearly changes priorities, and casts a new light on the political positions and actions of a number of states, and the need for cooperation as governments look to protect their peoples. I am hopeful this is an opportunity to recalibrate – in a variety of ways and disputes – what kind of a world we want going forwards.

“Among other things, this should not be one where cases like Nazanin become normalised in disputes between Iran and the UK. That is not the world I would like our daughter Gabriella to inherit as she comes to make sense of both parts of her heritage. I hope this is a chance to move on.”

He urges a resolution by 1 June at the latest, but adds he “remains guarded that we may still be being used by the Revolutionary Guard as a new kind of bargaining chip – as leverage in the campaign to challenge US sanctions. Given the ankle tag, given the mixed messages on how hard it was to get her temporarily released, it is not clear all parts of the regime are ready to let her go.”

He added: “This week we passed four years since Nazanin’s abrupt arrest. That is a long time for anyone to be held as collateral in someone else’s fight, but especially for the mother of a young child. So we are determined not to let this moment settle into a new kind of bargaining chip game.”