London (CNN) Britain's beleaguered Prime Minister Theresa May has been urged to sack her Foreign Secretary Boris Johnson over his handling of the Nazanin Zaghari-Ratcliffe case.

On Sunday both Labour leader Jeremy Corbyn and London Mayor Sadiq Khan called on May to fire Johnson -- who has been under increasing pressure since Iranian state TV claimed that his statements on the plight of the British-Iranian woman imprisoned in Tehran on charges of espionage confirmed her guilt.

Writing in the Observer on Sunday, Corbyn took aim at Johnson, calling for him to be relieved of his duties after "undermining our country" and "putting our citizens at risk."

Khan echoed those sentiments, telling the BBC that if "May was a strong Prime Minister she would have sacked him a long time ago."

Also on Sunday, the UK Foreign Office told CNN that Johnson had held a "constructive" conversation with Richard Ratcliffe, the husband of Zaghari-Ratcliffe.

Zaghari-Ratcliffe was arrested in 2016.

Johnson has been on the defensive over the past week since being forced to correct remarks he made to a British parliamentary committee, when he said that Zaghari-Ratcliffe, who has been in jail since she was detained at Tehran airport in April 2016, had been teaching journalism during her visit to the country. Zaghari-Ratcliffe and her family have maintained that she was in Iran visiting family.

After Johnson's original comments, authorities in Iran filed further charges against Zaghari-Ratcliffe, prompting fears her five-year sentence could be extended. On Wednesday, Iranian state TV IRIB said Johnson amounted to an unintentional confession.

Foreign Secretary Boris Johnson is set to visit Iran before the end of the year.

In his article, Corbyn says that the "heartbreaking" case of Zaghari-Ratcliffe has left her future liberty in peril because of Johnson's "serial bungling."

In a blunt final paragraph, Corbyn added: "We've put up with him embarrassing and undermining our country through his incompetence and putting our citizens at risk for long enough. It's time for Boris Johnson to go."

Khan was similarly critical of Johnson, his predecessor as mayor of London calling on him to "clarify the huge error he made" and help bring Zaghari-Ratcliffe back to the UK.

Zaghari-Ratcliffe with her daughter Gabriella.

Zaghari-Ratcliffe, who is being held on allegations relating to espionage, has denied the charges, always maintaining that she was in Tehran to visit family and was not working in the country at the time of her arrest.

She was detained at the airport in Tehran in April 2016 on her way back to the UK from visiting family with Gabriella, her then 22-month-old daughter.

The Iranian government accused her of working for a UK media network involved in activities against Iran. She was sentenced to five years in jail and her child was placed in the care of her parents.

Last month, Iran's Islamic Revolutionary Guard Corps (IRGC) opened a new case against her, accusing her of having joined organizations specifically working to overthrow the regime. She was also charged with having attended a demonstration outside the Iranian Embassy in London.

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Further tension

On Sunday, Johnson's colleague Michael Gove, the UK Environment Minister, added yet more uncertainty during an interview with the BBC.

Asked what Zaghari-Ratcliffe was doing in Iran when she was detained in 2016, he answered: "I don't know. One of the things I want to stress is that there is no reason why she should be in prison in Iran as far as I know."

When told that Richard Ratcliffe said his wife was in Iran on holiday, Mr Gove added: "In that case, I take exactly her husband's assurance in that regard."

Michael Gove is the UK Environment Secretary.

Questioned as to whether she had been training journalists, he said: "Her husband said she was there on holiday and her husband is the person who should know."

Gove's answers are likely to create yet more ammunition for the government's critics over its handling of the case.

Last week, amid an outcry from Zaghari-Ratcliffe's husband and fellow lawmakers, Johnson held a call with his Iranian counterpart Javad Zarif in an attempt to clarify his remarks and call for her release. He admitted that his comments to the parliamentary committee "could have been clearer," and added that he was "sorry" if his remarks were misconstrued.

During the call, Johnson "expressed concern at the suggestion from the Iranian Judiciary High Council for Human Rights that his remarks last week at the Foreign Affairs Committee 'shed new light' on the case," the UK Foreign Office said in a statement.

May under pressure

The calls for Johnson's dismissal come with May facing a crisis over her Cabinet after losing two ministers this month.

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Just last week, Priti Patel was forced to resign as secretary of state for international development following revelations she had met senior Israeli officials during a family holiday in the summer, a significant breach of diplomatic protocol.

Patel's resignation came just over a week after Defense Secretary Michael Fallon quit after becoming embroiled in the Westminster sexual harassment scandal.

There is also speculation over May's de facto deputy, Damian Green, was forced to deny allegations that "extreme" pornographic material was found on his work computer in 2008.

May's leadership is under increasing pressure with her Cabinet reeling from two resignations in the past week and her party's failure to win an overall majority in June's general election.

She is also facing criticism over the UK's handling of the Brexit negotiations with the government being called upon to release secret documents detailing the economic impact of withdrawing from the European Union.