Nazanin Zaghari-Ratcliffe, the British woman jailed by Iran on spy charges, has begun a new hunger strike at Tehran’s Evin Prison.
The protest came as her daughter Gabriella marked her fifth birthday, separated from both her mother and father.
Richard Ratcliffe and a crowd of supporters gathered outside the Iranian embassy in London to sing Happy Birthday to his daughter.
He acknowledged that it was “not a good time to be doing a hunger strike”.
“It's all quite fraught in the Gulf,” Mr Ratcliffe said. “As things get more tense we worry for our own situation, we worry that we'll get forgotten but also that things will get too tense for a solution to be found."
Britain’s Foreign Minister Jeremy Hunt, who is campaigning to become the UK’s next prime minister, met Mr Ratcliffe on Saturday.
In a Tweet, he urged Iran to “do the right thing” and “let this innocent woman home”.
The 40-year-old aid worker was arrested in April 2016 and has always maintained her innocence.
Meanwhile, Gabriella is being cared for by her grandparents in Iran.