The family of an Iranian-British woman serving a five-year prison sentence in Tehran for allegedly plotting to topple Iran's clerical establishment says a new case has been opened against her.

Nazanin Zaghari-Ratcliffe's family said that the charity worker faced a court hearing at Tehran's Evin prison on October 8 during which she heard of the new charges being brought against her.

An October 9 statement said that the new charges included 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 that Zaghari-Ratcliffe rejected the new charges, which could carry a sentence of 16 additional years in prison.

Her husband, Richard Ratcliffe, said the Islamic Revolutionary Guards Corps (IRGC) was "inventing new charges to prevent her early release."

The British Foreign Office said that it was "concerned following reports" about Zaghari-Ratcliffe's hearing.

"We continue to be concerned for the welfare of Mrs. Zaghari-Ratcliffe and have repeatedly raised this with the Iranian authorities," it said in a statement.

Zaghari-Ratcliffe is a project manager with the Thomson Reuters Foundation -- a charity organization that is independent of Thomson Reuters and operates independently of Reuters News -- and she previously worked for the BBC in London.

The foundation's chief executive, Monique Villa, called Zaghari-Ratcliffe's treatment by Iranian authorities "harrowing" and demanded that "these ludicrous charges must be dropped immediately."

Zaghari-Ratcliffe was arrested by the IRGC in April 2016 at Tehran's airport as she was about to return to Britain with her 2-year-old daughter after a family visit.

She was convicted of plotting to overthrow Iran's clerical establishment and sentenced to five years in prison in September 2016.

Her family and the Thomson Reuters Foundation denied the charges.

Zaghari-Ratcliffe is one of several dual nationals held in Iran by hard-liners in the country's judiciary and security services accused of violating national security. Iran does not recognize dual citizenship, which means they cannot receive diplomatic protections such as consular visits.

With reporting by Reuters and AP