The UK has branded the detention of Nazanin Zaghari-Ratcliffe for a further year in Iran as “totally inhumane and wholly unjustified”.

The British-Iranian charity worker has been given an additional jail term having already completed a five-year sentence on charges levied by Iranian authorities, the last year of which was spent under house arrest due to the pandemic.

Her lawyer Hojjat Kermani said she received the second sentence on a charge of spreading “propaganda against the system” for participating in a protest in front of the Iranian Embassy in London in 2009.

As well as the one-year jail term she has also been banned from leaving the country for a year.

Prime Minister Boris Johnson said on social media that the decision to sentence Mrs Zaghari-Ratcliffe to another year in prison was “cruel, inhumane and wholly unjustified”.

He tweeted: “Iran’s decision to sentence Nazanin Zaghari-Ratcliffe to another year in prison is cruel, inhumane and wholly unjustified. She must be allowed to return to her family in the UK and we will continue to do all we can to get her home.”

His comments were echoed by Foreign Secretary Dominic Raab, who said in a statement: “This is a totally inhumane and wholly unjustified decision.

“We continue to call on Iran to release Nazanin immediately so she can return to her family in the UK. We continue to do all we can to support her.”

Mr Johnson also said that the Government will be “working very hard” to secure her release, telling reporters: “Obviously we will have to study the detail of what the Iranian authorities are saying.

“I don’t think it is right at all that Nazanin should be sentenced to any more time in jail.

“I think it is wrong that she is there in the first place and we will be working very hard to secure her release from Iran, her ability to return to her family here in the UK, just as we work for all our dual national cases in Iran.

“The Government will not stop, we will redouble our efforts, and we are working with our American friends on this issue as well.”

In a statement, her MP Tulip Siddiq, who represents Hampstead and Kilburn, said: “This is a terrible blow for Nazanin and her family, who have been hoping and praying that she would soon be free to come home.

“It is devastating to see Nazanin once again being abusively used as bargaining chip.

“We’ve been told the Government has been working behind the scenes to secure Nazanin’s release.

“These efforts have clearly failed and we deserve an urgent explanation from minsters about what has happened.”

Mrs Zaghari-Ratcliffe was detained in 2016 on charges of crimes related to national security, and was sentenced to five years’ imprisonment.

She completed the latter part of her sentence under house arrest due to the coronavirus crisis in March, but was returned to court later in the month where she was tried on new charges of “spreading propaganda against the regime”.

Redress, the human rights group supporting Mrs Zaghari-Ratcliffe, said her lawyer in Iran intends to appeal the decision.

Its director Rupert Skilbeck said she had never received a fair trial in Iran, and was innocent of the allegations made against her.

He added: “Nazanin has already suffered severe physical and psychological impacts from the torture and ill-treatment she has been subjected to during the past five years.

“A further sentence to prison or house arrest may cause irreparable damage to her health.

“Nazanin has never received a fair trial in Iran, and is innocent of the allegations made against her.

“Her detention has always been illegal under international law.

“The case must be dismissed and she should be allowed to return to her husband and daughter in the UK immediately.”

Some observers have linked Mrs Zaghari-Ratcliffe’s case to a long-standing debt Iran alleges it is owed by the UK.

The UK is thought to owe Iran as much as £400 million over the non-delivery of tanks in 1979, with the shipment stopped because of the Islamic revolution.

Ministers have said Britain continues to “explore options” to resolve the dispute, but insisted the “two issues cannot be merged into one”.

Former foreign secretary Jeremy Hunt questioned why the issue of an IMS (International Military Services) debt had not been dealt with.

He tweeted: “This is so distressing. Iran’s cruelty seems to know no bounds.

“Impossible to imagine what the family are going through today.

“Key question is why the IMS debt issue is still not settled given the UK accepts that it owes this money?”

The new jail term comes amid tensions in the Middle East over Iran’s nuclear programme, with the country abandoning all limits of its 2015 nuclear deal with world powers in the wake of former US president Donald Trump’s 2018 decision to unilaterally withdraw from the accord.

US President Joe Biden has expressed a desire to return to the deal if Iran honours limits on its nuclear programme.

During a call with Iranian president Hassan Rouhani last month, Downing Street said Mr Johnson had raised Mrs Zaghari-Ratcliffe’s case and Iran’s breaches of its nuclear deal, the Joint Comprehensive Plan of Action (JCPOA).

A Number 10 spokesman said Mr Johnson had “stressed that while the UK remains committed to making the Iran nuclear deal a success, Iran must stop all its nuclear activity that breaches the terms of the JCPOA and come back into compliance”.