Nazanin Zaghari-Ratcliffe was sentenced on Monday to an additional year in prison after being found guilty of propaganda offences against Iran.
The British-Iranian aid worker was also banned from leaving the country for one year, meaning she won't be able to return to her family in the UK.
Her lawyer Hojjat Kermani said the latest sentence related to a charge of spreading “propaganda against the system” by participating in a protest in front of the Iranian Embassy in London in 2009.
Redress, a human rights charity, told The National Ms Zaghari-Ratcliffe's Iran-based lawyers were summoned on Monday and shown the verdict.
UK Prime Minister Boris Johnson confirmed a further extension of an ordeal that saw her detained five years ago while on a visit to her parents. Mr Johnson said on Monday the sentence was "cruel, inhumane and wholly unjustified".
"I don't think it is right at all that Nazanin should be sentenced to any more time in jail, I think it's wrong that she's there in the first place," he said.
"We will be working hard to secure her release – the government will not stop, we will redouble our efforts. I have to say we are working with our American friends on this issue as well."
Foreign Secretary Dominic Raab said the sentence was 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," he said.
Her husband Richard Ratcliffe told the BBC the court's decision was "clearly a negotiating tactic" by the Iranian authorities as discussions over the country's nuclear activities continued.
The family has long argued that Ms Zaghari-Ratcliffe was being held as a bargaining chip by Iranian authorities over what Tehran says is £400 million ($554.9m) debt owed by the UK to Iran for the non-delivery of Chieftain tanks.
The debt is subject to legal proceedings in UK courts.
Former foreign secretary Jeremy Hunt questioned why the issue of an IMS (International Military Services) debt had not been dealt with.
"This is so distressing. Iran’s cruelty seems to know no bounds. Impossible to imagine what the family are going through today," he tweeted.
“Key question is why the IMS debt issue is still not settled given the UK accepts that it owes this money?”
Mr Kermani said he would appeal the sentence for participation in propaganda activities against Iran. It is not known if Ms Zaghari-Ratcliffe will be forced to return to jail from her parents' house in Tehran.
Verdict risks 'irreparable damage' to Nazanin Zaghari-Ratcliffe's health
The mother-of-one has been detained in Iran since 2016 and completed a five-year sentence for allegedly being a spy. The 42 year old has always denied the claims.
She was released from jail in March 2020 as the Covid-19 pandemic hit the Middle East. She was ordered to wear an electronic tag while staying at her parents' house in Tehran.
But she returned to court later that month to face the latest propaganda charges.
Redress said it was not yet clear how Ms Zaghari-Ratcliffe would be expected to serve the sentence given that she has already spent a year under effective house arrest.
The London-based charity said the propaganda charge was introduced in 2017 and "invoked inconsistently" since, with the evidence remaining unclear.
Rupert Skilbeck, the organisation's director, said Ms Zaghari-Ratcliffe should be allowed to return to the UK immediately.
“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," he said.
“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."
Amnesty International UK director Kate Allen said the "sham trial" was "further proof of the incredible cruelty of the Iranian regime".
"We fear that going back to jail will be almost too much for Nazanin to bear," she said.
Medical experts said in March that Ms Zaghari-Ratcliffe required urgent psychiatric care for depression and post-traumatic stress disorder because of the conditions she experienced in a Tehran jail.
She suffered from hair loss and obsessive compulsive disorder, and was forced to hear a female prison guard talking to her daughter repeatedly at a time when she was already highly distressed about being separated from her child.