Jeremy Corbyn has fired an extraordinary broadside against Boris Johnson, calling for him to be sacked immediately as foreign secretary for “undermining our country” and “putting our citizens at risk”.

The blistering attack – and demand that Theresa May fire him – was delivered exclusively in a statement to the Observer on Saturday night, as pressure mounted on Johnson over his diplomatic blunder in the case of Nazanin Zaghari-Ratcliffe, the British mother imprisoned in Iran.

The Labour leader cites a litany of undiplomatic and ill-chosen statements from Johnson since his appointment by May as foreign secretary in July last year.

Corbyn accuses him of having a “colonial throwback take on the world”, and of repeatedly “letting our country down”.

It is the mishandling of the “heartbreaking” case of Zaghari-Ratcliffe that persuaded Corbyn to call for his dismissal. His statement ends: “We’ve put up with Johnson embarrassing and undermining our country with his incompetence and colonial throwback views and putting our citizens at risk for long enough. It’s time for him to go.”

The intervention places both May and Johnson under renewed pressure after 10 days in which the prime minister has been forced to dismiss defence secretary Sir Michael Fallon for inappropriate behaviour towards women, and the international development secretary, Priti Patel, for conducting a freelance aid policy in the Middle East without informing No 10 or the Foreign Office.

After a succession of scandals and crises, Johnson’s future now looks increasingly in doubt as Tehran considers whether to increase Zaghari-Ratcliffe’s five-year sentence imposed for plotting to undermine the Iranian state.

The Brexit secretary David Davis, was due to defend Johnson on the Sunday with Niall Paterson programme on Sky News. Asked whether Johnson is “unsackable”, Davis replies: “Why would you want to sack him? He’s a good foreign secretary. The thing to understand here – you’ve got a flurry at the moment of things happening in politics, but this happens to all governments at some point or other.”

Zaghari-Ratcliffe, a 38-year-old project manager with the Thomson Reuters Foundation, has been in prison since her arrest in Tehran in April last year.

Nazanin Zaghari-Ratcliffe with her husband Richard Ratcliffe and their daughter Gabriella. Photograph: PA

Pressure has been building on Johnson since he told the foreign affairs select committee earlier this month that he believed she had been “simply teaching people journalism, as I understand it”.

Her family were outraged by the comments, saying they were entirely inaccurate and place her at further risk. They insist Zaghari-Ratcliffe was on holiday. Her employers say she had not taken part in the training of journalists.

Iranian state-run TV bulletins have, however, referred to Johnson’s comments as evidence she was acting illegally. “The sole sentence uttered from the mouth of the UK foreign secretary put the efforts of the British media [propaganda] in vain,” said one.

The Foreign Office has said Johnson’s remarks would have no impact on her imprisonment, although he has said his comments “could have been clearer”.

Zaghari-Ratcliffe’s husband, Richard Ratcliffe, has said he wants to meet Johnson and join him on a trip to Tehran. He said the Foreign Office made contact with him on Friday, two days after Johnson said he was willing to meet him for the first time since his wife’s arrest.

Iran’s supreme court upheld the conviction on undisclosed national security charges and a final appeal was lodged in January.

Homa Hoodfar, a ​Canadian-Iranian professor of social anthropology who shared a cell with Zaghari-Ratcliffe for a night in June 2016, told the Observer of the stress she was under.

The encounter was around the time of Zaghari-Ratcliffe’s daughter’s birthday and she had been promised that she would be released if she signed some documents. “Because of that promise,” said Hoodfar, “she signed a series of [confession] documents that she shouldn’t have. Nazanin was under a lot of stress.”

A Foreign Office spokesperson said: “We remain very concerned about all our dual nationals detained in Iran and are doing everything we can in each of their cases, including trying to secure access to them and ensure their welfare. We will continue to do this in the way that we judge is in their best interests.”