Nazanin Zaghari-Ratcliffe has been sentenced to a further year in prison and a one-year travel ban after being found guilty of propaganda against the regime in Iran.
Her lawyer said she was accused of taking part in a protest in London 12 years ago and speaking to the BBC Persian service.
The prime minister said the UK would “redouble” efforts to free her.
The British-Iranian charity worker was first jailed in Tehran in 2016.
She has always denied the spying charges levelled against her.
Confirming the latest sentence, her husband, Richard Ratcliffe, said the court’s decision was a bad sign and “clearly a negotiating tactic” by the Iranian authorities – who are in the middle of discussions over the country’s nuclear activities.
Mrs Zaghari-Ratcliffe has not been taken to prison yet, her husband said, and plans to appeal against the sentence.
“The threat is there, and the threat is bigger than we were fearing.” Mr Ratcliffe told the BBC. Referring to the possibility of being separated from his wife until summer 2023, he said: “I think the worst case got a bit closer.”
Mr Ratcliffe has not seen his wife in person since her imprisonment in 2016. Their daughter, Gabriella, who was with her mother in Tehran when she was arrested, has been with him in the UK since 2019.
Prime Minister Boris Johnson said: “I don’t think it is right at all that Nazanin should be sentenced to any more time in jail.”
He said it was “wrong that she is there in the first place” and pledged to work hard to secure her release so she could return to her family, “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,” Mr Johnson said.
Mr Ratcliffe maintains his wife was imprisoned as leverage for a debt owed by the UK over its failure to deliver tanks to Iran in 1979.
He says the case may also be caught up in negotiations over the international agreement to limit Iran’s enrichment of nuclear material, which the UK and others are trying to revive.
The sentencing may mean that part of the Iranian regime is unhappy with the direction of the negotiations taking place in Vienna and is signalling that things can get worse for Mrs Zaghari-Ratcliffe and other jailed dual nationals, Mr Ratcliffe said
Last year, Mrs Zaghari-Ratcliffe was moved from prison due to the coronavirus crisis and held under house arrest in Tehran until March, when her ankle tag was removed.
But she returned to court later that month to face the latest propaganda charges.