The husband of Nazanin Zaghari-Ratcliffe, a British-Iranian woman detained in Tehran, on Wednesday called on the British government to do "everything to protect her" after Iran announced that she faces fresh charges.

Zaghari-Ratcliffe has already spent more than four years in jail or under house arrest since she was detained at Tehran airport on sedition charges in April 2016 after visiting relatives in Iran with her young daughter.

On Tuesday, Iranian state television's website Iribnews reported: "The 15th chamber of the Islamic Revolutionary Court summoned Nazanin Zaghari this morning with her lawyer... to notify them of a new indictment".

The report cited "an informed source" but gave no further details on the charges or trial date.

Her husband, Richard Ratcliffe, said in a statement that "it has become increasingly clear the past months that Nazanin is a hostage, held as leverage against a UK debt".

"It is important that the UK government does everything to protect her and others as Iran's hostage diplomacy continues to escalate," he added.

"This starts with the British embassy insisting it is able to attend Nazanin's trial on Sunday."

Nazanin Zaghari-Ratcliffe, pictured with her daughter in 2018 has been on temporary release from Evin prison in Tehran and under house arrest since earlier this year because of the coronavirus pandemic
Nazanin Zaghari-Ratcliffe, pictured with her daughter in 2018 has been on temporary release from Evin prison in Tehran and under house arrest since earlier this year because of the coronavirus pandemic Free Nazanin campaign / -

Zaghari-Ratcliffe, who at the time worked for the Thomson Reuters Foundation -- the media organisation's philanthropic arm -- was convicted on sedition charges, which she denies, and sentenced to a five-year jail term.

Tulip Siddiq, the British MP for Zaghari-Ratcliffe's constituency, tweeted that she had been in touch with the detainee, and confirmed that "she was taken to court this morning and told she will face another trial on Sunday".

Britain's foreign ministry said bringing new charges was "indefensible and unacceptable".

"We have been consistently clear that she must not be returned to prison," a ministry spokesperson said.

Zaghari-Ratcliffe has been on temporary release from Evin prison in Tehran and under house arrest since earlier this year because of the coronavirus pandemic.

Possible links have been drawn in the UK and Iran between Zaghari-Ratcliffe's detention and a British debt dating back more than 40 years, to when the shah of Iran paid the UK ?400 million for 1,500 Chieftain tanks.

When the shah was ousted in 1979, Britain refused to deliver the tanks to the new Islamic Republic but kept the money.

Coronavirus United kingdom