Richard Ratcliffe said there is a lack of medicine, disinfectant, bleach and antiseptic in the Evin prison in Tehran and conditions have been “worsening” over the past few months.
Writing in the i newspaper on Friday, he said he fears she is at risk of contracting Covid-19 as she “has a severe sore throat” and “woke up shivering with cold”.
Mr Ratcliffe noted: “For prisoners, there is no antiseptic, no disinfectant, no bleach until next month. Though for the guards there is still a constant supply of antiseptic soap and sanitary gel, to make sure they stay safe.
“The threat of coronavirus in Evin prison seems much more severe because of the wider health conditions.”
He called on Boris Johnson, the prime minister, to take action on securing her release, adding his wife already suffered from other medical conditions as a “direct consequence of her time in prison”.
Ms Zaghari-Ratcliffe, who has been imprisoned in Tehran since 2016, experiences “unexplained collapses to irregular heartbeats, to strange incapacities in her neck and arms,” her husband said.
Earlier this week, Tory MP Jacob Rees-Mogg said he would “not always take as authoritative” the Iranian authorities’ denial of any outbreak of the virus at the prison.
But on Thursday, the Iranian judiciary said it was considering plans to grant eligible prisoners leave “to reduce inmates’ numbers in prisons and limit the spread of coronavirus”.
The plans were made following reports a prisoner there had died after suffering from coronavirus symptoms, the Middle East Monitor reported.
“From far away it is hard to assess the risk calmly in Nazanin’s case – knowing we are not being told the truth is not the same as knowing what the truth is. But this week the panic grew,” Mr Ratcliffe wrote.
He said eligibility for temporary release does not extend to hostages, which his wife is, and fears quarantine measures may “become another solitary confinement”.
“This week, we are watching carefully, hoping a cold is just that,” he said. “Right now we don’t want the government to watch on, but to act.
“Nazanin could have been released years ago, if promises had been kept. Just how far does this need to go.”
Caroline Lucas, former leader of the Green Party, weighed in on Twitter: “The detention of Nazanin Zaghari-Ratcliffe in Evin prison in Iran is a travesty of justice.
“Now she’s at risk of coronavirus, with no access to medicine and worsening conditions in the prison. What is the UK government doing to secure her release?”
Mr Johnson has been widely criticised for his handling of her imprisonment after he mistakenly said Ms Zaghari-Ratcliffe had been “teaching people journalism” in the country while he was foreign secretary in 2017.
She was arrested and accused of spying and plotting to overthrow the Iranian government while on a trip to visit her family in Iran in 2016 and has remained in prison since.
In December last year, Mr Ratcliffe told the prime minister to “stop getting distracted” in the fight to release her and accused the government of “enabling the abuse of British citizens”.