LONDON (Reuters) - Iranian authorities have opened a new case against a British-Iranian charity worker serving five years in jail in Tehran, leveling charges that could carry a sentence of 16 additional years in prison, her family said on Monday.

Nazanin Zaghari-Ratcliffe and her daughter Gabriella pose for a photo in London, Britain February 7, 2016. Picture taken February 7, 2016. Karl Brandt/Courtesy of Free Nazanin campaign/Handout via REUTERS

Nazanin Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, was arrested by the elite Revolutionary Guards in April 2016 at a Tehran airport, as she was about to return to Britain with her two-year-old daughter after a family visit.

She was convicted of plotting to overthrow Iran’s clerical establishment, a charge denied by her family and the Foundation, a charity organization that is independent of Thomson Reuters and operates independently of Reuters News. She was sentenced to five years in prison.

In a hearing in Evin Prison, where she is serving her term, Zaghari-Ratcliffe was told on Sunday that a new case had been opened at the insistence of the Revolutionary Guards, Iran’s most powerful security force, her family said in a statement.

The new charges include joining and receiving money from organizations working to overthrow the Islamic Republic, and attending a demonstration outside the Iranian Embassy in London, the family said.

“Nazanin and the rest of her family are bewildered at this turn,” the family statement said.

Her husband Richard Ratcliffe said the Revolutionary Guards were “inventing new charges to prevent her early release”.

Iran’s judiciary was not immediately available for comment.

Zaghari-Ratcliffe used to work in London for the BBC, which Iran says is seeking to topple the Islamic theocracy. During Sunday’s hearing she rejected all the new charges, the family said.

Iran refuses to recognize dual nationals and denies them access to consular assistance.

On the sidelines of the U.N. General Assembly in September, Britain’s foreign minister, Boris Johnson, said progress in the relationship between London and Tehran would depend to some extent on a resolution of the Nazanin case.