Ms Zaghari-Ratcliffe, a dual British-Iranian citizen, has been imprisoned in Tehran since 2016, when she was arrested and accused of spying during a trip to see her family in Iran.
Mr Johnson was widely criticised for his handling of her case when he was the foreign secretary in 2017 after he mistakenly said Ms Zaghari-Ratcliffe had been “teaching people journalism” in the country. He was later forced to apologise.
Richard Ratcliffe, her husband, has accused the UK of “enabling the abuse of British citizens” and said Mr Johnson’s government “needs to take a much more robust stand against Iranian hostage-taking”.
Ms Zaghari-Ratcliffe is due to go on hunger strike in solidarity with a British-Australian academic who has been sentenced to 10 years in prison on spying charges, according to Mr Ratcliffe.
Kylie Moore-Gilbert, the academic, started an open-ended hunger strike before Christmas and has called on Scott Morrison, Australia’s prime minister, to do more to secure her release.
“If President Trump can get Americans home with all the baggage there, then there is no earthly reason why the UK cannot,” Mr Ratcliffe said, referring to successful negotiations in recent years to secure the release of US hostages.
“It is a question of priorities, and the prime minister not getting distracted again.”
He added that Ms Zaghari-Ratcliffe, who also refused food on Christmas Eve, told him on Monday: “I really can’t keep going on like this, someone needs to do something.”
“When people are desperate, a hunger strike is the only weapon people have,” Mr Ratcliffe said.
“It is to get the Iranian regime to notice, but it is also to cut through the complacency of their own governments, to remind them of their promises to get things done or leave no stone unturned.”
In 2017, Mr Johnson’s comment about Ms Zaghari-Ratcliffe teaching journalism was cited as evidence against her when she appeared in an Iranian court.
A spokesperson for the Foreign Office said the department remains “extremely concerned about the welfare of Ms Zaghari-Ratcliffe and other British-Iranian dual nationals detained in Iran.”
“We continue to raise her case at the highest levels, are in regular contact with her family and press Iran for her release,” they added.
Mr Ratcliffe also expressed his support for Ms Moore-Gilbert and called on the Australian and British governments to “step up” their efforts to secure her release.
“Five days on dry hunger strike is becoming critical, and our thoughts go out to Kylie and her family for all this ordeal,” he told The Guardian on Sunday.
Additional reporting by PA