Nazanin Zaghari-Ratcliffe, the British-Iranian woman jailed in Iran, has been told she has to face another trial.
The charity worker is nearing the end of her five-year sentence for spying charges, which she has always denied.
Iranian state media said she was brought before a revolutionary court in the country's capital Tehran on Tuesday morning.
The Foreign Office said the new charge, which has not been made public, was "indefensible and unacceptable".
Mrs Zaghari-Ratcliffe was arrested in April 2016 while travelling to visit her parents with her young British-born daughter, Gabriella.
The dual national was sentenced to five years in prison over allegations of plotting to overthrow the Iranian government, which she denies.
Earlier this year, she was given temporary leave from prison because of the coronavirus outbreak and has been living at her parents' house with an ankle tag.
Before her arrest, Mrs Zaghari-Ratcliffe lived in London with her husband Richard.
Her MP, Tulip Siddiq, said she had spoken to Mrs Zaghari-Ratcliffe. "I've been in touch with Nazanin Zaghari-Ratcliffe and can confirm that she was taken to court this morning and told she will face another trial on Sunday," she said.
"I know many people are concerned about her welfare and I'll keep everyone updated when we have more information.
"This is an extremely worrying development, and I know many people are concerned about Nazanin's welfare."
The MP added: "The last four years have been excruciating for her husband Richard and her daughter Gabriella, who is growing up without a mother.
"The United Nations have recognised Nazanin's imprisonment as arbitrary and unlawful, and any further court case is clearly unacceptable."
Debt to Iran
Mr Ratcliffe has previously expressed fears that she could face a second court case when her sentence ends.
He said she and other dual nationals are being held hostage because Iran wants the UK to pay a decades-old debt over an arms deal that was never fulfilled.
The human rights group, Amnesty International, said it appeared Iranian authorities were "playing cruel political games with Nazanin".
"Nazanin has already been convicted once after a deeply unfair trial, and there should be no question of her being put through that ordeal again," the charity's UK director Kate Allen said.
"As a matter of absolute urgency the UK government should make fresh representations on Nazanin's behalf, seeking to have any suggestion of a second trial removed."
The Foreign Office said in a statement: "Iran bringing new charges against Nazanin Zaghari-Ratcliffe is indefensible and unacceptable.
We have been consistently clear that she must not be returned to prison."
Former Foreign Secretary Jeremy Hunt tweeted: "Nazanin has already served most of her sentence for a crime she didn't commit.
"This is hostage diplomacy and Iran needs to know that Britain will not stand for it."
Mrs Zaghari-Ratcliffe's family and the UK government have always maintained her innocence and she has been given diplomatic protection by the Foreign Office - meaning the case is treated as a formal, legal dispute between Britain and Iran.