The husband of a British-Iranian woman imprisoned in Iran has criticised the UK Government for failing to publicly call for her release, saying the way the Foreign Office has behaved in relation to his case has been “deeply secretive and deeply cryptic”.

Iranian authorities announced on Sunday that Nazanin must serve five years in prison after she was sentenced in September last year for allegedly plotting to topple the Iranian government.

The exact details of the charges have not been made public.

The 37-year-old London-based charity worker was seized by Iranian Revolutionary Guards on 3 April as she was travelling back to Britain with her two-year-old daughter Gabriella after visiting family.

Supporters hold a photo of Nazanin Zaghari-Ratcliffe during a vigil (Getty)

Iran’s Deputy Head of the Judiciary said at a weekly news conference that the appeal court had upheld her sentence of five years for national security related crimes.

Speaking to the Independent, Richard Ratcliffe said his wife’s detention was politically motivated and repeated claims she was being held as a bargaining chip in an outstanding arms debt stretching back 40 years that Britain is refusing to pay.

The 42-year-old accountant said the British Foreign Office officials he had worked with on trying to secure his wife’s release were “decent people and perfectly sincere”.

But he questioned why the family had not been offered a meeting with the Foreign Secretary Boris Johnson to put further pressure on Tehran.

And he criticised the Government for failing to publicly condemn the imprisonment of his wife, who went on hunger strike late last year after “feeling suicidal”.

"They have been deeply secretive and deeply cryptic," he said.

“It is clear there are other agendas in the relationship the Foreign Office has with Iran.

“We are not irrelevant but there are other considerations. They have never expressed any criticism, and they have never called for her release publicly.”

The family hope she will be pardoned by Iran’s Supreme Leader but Mr Ratcliffe said this was unlikely to happen before the presidential election scheduled for 19 May.

He added: "When we get to Ramadan in June that is when the realistic chances of release might happen, so I need to be campaigning pretty hard by then as they are not backing down."

Iran does not recognise dual nationalities, meaning those detained cannot receive consular assistance.

In most cases, dual nationals have faced secret charges in closed-door hearings before Iran's Revolutionary Court, which handles cases involving alleged attempts to overthrow the government.

Prime Minister Theresa May raised concerns about Ms Zaghari-Ratcliffe's case during a phone call with Iranian President Hassan Rouhani last August.

But she remains in an Iranian jail, having served 10 months behind bars so far, two months of which, Mr Ratcliffe said, have been in solitary confinement.

Their baby daughter is also in the country and being looked after by family.

Nazanin Zaghari-Ratcliffe with husband Richard and daughter Gabriella (Change.org)

A Foreign Office spokesman said: “We are concerned by reports Ms Zaghari-Ratcliffe’s sentence has been confirmed.

“The Prime Minister and Foreign Secretary have raised her case with their counterparts in Iran and will continue to do so.

“We have been supporting her family since we were first made aware of her arrest and the Minister for the Middle East, Tobias Ellwood, has met the family to reassure them that we will continue to do all we can.