British-Iranian Anoosheh Ashoori, who is being held in Iran on spying charges, is beginning a hunger strike.
The retired civil engineer, who is approaching his 68th birthday, has been detained in Evin prison for four years and denies the charges against him.
His daughter said he hoped the protest would bring "global attention to the plight" of those unfairly held by Iran.
The Foreign Office said Iran's continued detention of Mr Ashoori was "wholly unjustified".
Mr Ashoori's daughter, Elika Ashoori, announced the hunger strike on Twitter.
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Mr Ashoori lived in the UK for 20 years, but was arrested in Iran after flying out to see his mother in 2017. He was convicted in July 2019 of spying for Israel's Mossad intelligence agency.
In April 2021, Mr Ashoori's legal team submitted a request to the UK government to grant him diplomatic protection, but this has not yet been granted.
His wife, Sherry Izadi, has told the BBC she is concerned for his health as he is on prostate and cholesterol medication.
His family have said that during his detention, Mr Ashoori has been subjected to solitary confinement, coercive and abusive interrogations without a lawyer, forced confession, and threats to his and his family's safety.
They added that Mr Ashoori has been forced to represent himself during his trial and is in an extremely vulnerable position.
Like Nazanin Zaghari-Ratcliffe, he is one of about a dozen Western dual nationals being held by Iran.
As well as drawing attention to his own plight, Mr Ashoori wants to show solidarity with a 77-year-old former US diplomat who was held in Iran after the 1979 Islamic Revolution. Barry Rosen began a hunger strike last week in Vienna, where talks to revive a nuclear deal with Iran are being held.
Mr Rosen is demanding that no deal be made without the release of dual nationals from several Western countries being held by Tehran as what he calls bargaining chips.
Ms Ashoori said that her father would join others "who have courageously begun a hunger strike in Vienna, demanding that the release of all foreign and dual nationals held by Iran should be a pre-condition of any new agreement by Iran and the JCPOA parties".
The Joint Comprehensive Plan of Action (JCPOA) is known as the Iran nuclear deal. It is an agreement made in 2015 between Iran and seven world powers (the US, UK, France, China, Russia and Germany). Under the accord, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for the lifting of economic sanctions.
The parties involved in the nuclear deal are meeting in Vienna this month.
Amnesty International's Sacha Deshmukh said the UK government needed to "immediately grant Anoosheh diplomatic protection status" and should "use this as the basis for more sustained action to secure Anoosheh's release and the release of all UK nationals being arbitrarily detained in Iran".
A Foreign, Commonwealth and Development Office (FCDO) spokesperson told the BBC that "Iran's continued detention of Anoosheh Ashoori is wholly unjustified".
They added: "Iran should free all those British nationals unfairly detained in Iran. The Foreign Secretary, Minister Cleverly and senior officials consistently raise the cases of Nazanin Zaghari- Ratcliffe, Anoosheh Ashoori and Morad Tahbaz with the Iranian authorities and will continue to do so."
Mrs Zaghari-Ratcliffe has been detained in Iran for five years on spying charges and lost an appeal against a second jail sentence in October.
She was arrested there in 2016 while taking the couple's daughter, Gabriella, to see her family, and was accused of plotting to overthrow the government. She served four years of a five-year sentence in Evin prison in Tehran, and one under house arrest.