Retired civil engineer Anoosheh Ashoori, 67, has been held in Tehran's Evin prison for the past four-and-a-half years on charges of spying for Israel - allegations he denies.

It is believed he was snatched from the streets of Tehran while on a trip to see his mother.

Surviving in an Iranian prison - the story of Anoosheh Ashoori

His family, who live in the UK, want Mr Ashoori to be given "diplomatic protection" - like Nazanin Zaghari-Ratcliffe - but have said they found the process "really offensive".

They believe their loved one is in effect being held hostage as part of the dispute over the longstanding £400m debt the UK owes Iran.

The 67-year-old has written letters to Boris Johnson from his prison cell.

Mr Ashoori's daughter, Elika, has now shared a video on social media, saying her father is going on hunger strike in "solidarity" with others such as former US diplomat Barry Rosen - who was one of 52 Americans held hostage in Iran after the Islamic Revolution in 1979, and is currently demonstrating in Vienna - to demand Iran release all foreign and dual-national "hostages".

"My father, who has been held hostage by Iran for the past four-and-a-half years, has decided to join their hunger strike from Evin prison," Ms Ashoori said in the video posted on Twitter.

"Needless to say, we are extremely concerned for his physical health as he approaches his 68th birthday.

"But having failed to see any progress in the FCDO's [the foreign office] efforts to bring about his release and no sign that the welfare of hostages currently held by Iran is a priority of the respective US, Europe and UK governments, he will begin his hunger strike in the hope of bringing global attention to the plight of these individuals held by Iran.

"Please help us by sharing this message using the hashtag #FreeTheHostages."

Mr Rosen began his own hunger strike on Wednesday in Vienna, where there is pressure for countries to make the release of dual-nationality prisoners a condition for the resumption of the JCPOA (Joint Comprehensive Plan of Action) - the 2015 agreement for Iran to limit nuclear activities in exchange for economic sanctions being lifted.

The pact was made with the US, UK, France, China, Russia and Germany - but was suspended under Donald Trump's presidency.

An FCDO spokesperson said: "Iran's continued detention of Anoosheh Ashoori is wholly unjustified. We call on Iran to end his suffering and allow him to return home to be reunited with his wife and children.

"Iran should free all those British nationals unfairly detained in Iran. The foreign secretary, minister [James] 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."