The husband of Nazanin Zaghari-Ratcliffe has begun a hunger strike outside the Iranian embassy in London in solidarity with his wife. Richard Ratcliffe called for a meeting with the next prime minister and demanded the incoming government stand up for British citizens unjustly held abroad.

The British-Iranian woman started her third hunger strike against her continued imprisonment in Iran on Saturday. She had vowed to take action if she was still in jail on their daughter Gabriella’s fifth birthday. At the same time, a vigil got under way in front of the country’s embassy in Knightsbridge.

Zaghari-Ratcliffe, who denies charges of espionage in Iran, told her husband in a phone call on Saturday morning that she was beginning a hunger strike to demand her unconditional release. Ratcliffe immediately decided to join her and on Saturday afternoon began the fasting vigil, which he plans to keep up until his wife’s strike ends.

“Nazanin is determined to show the Iranian authorities that enough is enough,” Ratcliffe said. “She was quite calm on the phone, if nervous. My job is to serve as an amplifier and not let her strike pass unnoticed.”

Zaghari-Ratcliffe went on hunger strike for six days in 2016 when she was put in solitary confinement for eight months without access to a lawyer, and did so again in January this year.

She ended her second strike when her demands to have specialist medical assessments were met after three days, although she still has not received treatment for any of the diagnosed medical problems.

It will be Ratcliffe’s first strike: “I had promised Nazanin that I wouldn’t let her go through a strike alone again.”

His demands to the Iranian authorities are for his wife’s immediate release; an immediate visit by the British embassy to check on her health; and a visa for him to travel to Iran if her release is not granted. He has not seen his daughter, who was arrested for espionage along with Zaghari-Ratcliffe and now lives with her grandparents in Tehran, for 1,185 days.

Gabriella, a British citizen who was born in London, has since forgotten how to speak English. Ratcliffe said he would like to bring her home to London before she is due to start school in September, but would only do so with the agreement of his wife, who cherished her short prison visits with her daughter and had been feeling very “emotional” in the run-up to her birthday.

Richard Ratcliffe holds a photo of himself with Nazanin and Gabriella last year to mark the second anniversary of his wife’s detention. Photograph: John Stillwell/PA

Gabriella was almost two when Zaghari-Ratcliffe was arrested on charges of espionage during a family holiday to Iran in April 2016. On Saturday, friends, family and supporters sang happy birthday to her via FaceTime – after she turned five on Tuesday – and shared a birthday cake in the shape of a unicorn.

He and the families of other British citizens who have been imprisoned abroad wrote to all the Tory leadership candidates on Wednesday asking them to make a commitment to protect British citizens overseas better as a policy priority, but have not yet received a response from any of them.

“The call we’ve made to all of the candidates is that they make the protection of British citizens who are being held unfairly in prison and subjected to torture their top priority,” he said from outside the embassy. “That applies to Nazanin, and many others. The next prime minister should make it clear that their government is going to stand up for British citizens.”

He added: “Some of us have met the foreign secretary, but none of us have met the prime minister, so there’s levels of significance. Whoever’s going to be in charge, I want to meet them.”

The foreign secretary, Jeremy Hunt, tweeted on Saturday:

With Richard Ratcliffe this morning whose family have been separated for over 3 years. As Richard continues his campaign for Nazanin’s release at the Iranian embassy,my message to Iran: DO THE RIGHT THING, SHOW THE WORLD YOUR HUMANITY & LET THIS INNOCENT WOMAN HOME #freenazanin pic.twitter.com/bitSctNmn9 — Jeremy Hunt (@Jeremy_Hunt) June 15, 2019

Ratcliffe had taken a few changes of clothes with him to the embassy, he said, but was hoping the couple would not have to strike for weeks.

“I will stay outside this embassy so that Nazanin’s hunger strike will be unignorable,” he said. “This is a bit in their face, deliberately. We need to show how serious we are, and we’ll take it day by day.”

He was not expecting her to be freed in two days’ time. “My experience of watching her hunger strikes is that the Iranian authorities are fairly battle-hardened. I think they will try to wait it out and see how serious it gets. But Nazanin is giving them a clear message: this cannot go on,” he said, then repeated: “This just cannot go on.”

On the subject of the Tory leadership candidate Boris Johnson, who in 2017 made misleading statements about Zaghari-Ratcliffe’s work which were seized upon by the Iranian regime, he said: “Knowing the prime minister, whoever it is, would be a slight advantage to not knowing them. Obviously, there’s baggage. Clearly, mistakes happened and at points I got annoyed that mistakes were denied. But also, there’s more going on than just what Boris said in that select committee as to why Nazanin is still being held in prison. There’s a reason why we are camped in front of the Iranian embassy.”

A knitting group wrapped a “chain of care” around the lampposts outside the embassy, with each link containing 1,190 stitches to represent the number of days Zaghari-Ratcliffe will have spent in prison by 7 July. “We hope the Iranian authorities will take notice,” said Catherine Allison, from the West Hampstead knitting group.