Jeremy Corbyn has launched a furious attack on Boris Johnson, calling for the Foreign Secretary to be sacked for "putting our citizens at risk".
The Labour leader said his appeal was motivated by the "heartbreaking" case of Nazanin Zaghari-Ratcliffe, a British woman jailed in Iran.
He told The Observer that Mr Johnson was "undermining our country with his incompetence" by refusing to apologise for comments that could extend the detained Briton's sentence.
Mr Corbyn said the "embarrassing" behaviour, coupled with the Foreign Secretary's "colonial throwback" attitude meant he should resign.
"It's time for him to go," the Labour leader said.
It comes as the focus shifts from former International Development Secretary Priti Patel on to Mr Johnson, who said Mrs Zaghari-Ratcliffe had been "teaching people journalism"in Tehran, despite her family and employer's insistence she was on holiday.
The comment was reported as an "unintended confession" by Iranian state TV, and raised fears her jail term might increase as a result.
Mr Johnson at first refused to apologise, but later said he was sorry if his remarks about the British-Iranian mother caused her family anxiety.
Brexit Secretary David Davis defended his Cabinet colleague, telling Sky's Sunday with Niall Paterson: "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 'Continued on page five, continued from page one' moment of things happening in politics, but this happens to all governments at some point or other."
Mrs Zaghari-Ratcliffe's husband, Richard, has told Sky News he is in contact with senior politicians over the case, and is expected to meet Mr Johnson on Sunday.
Environment Secretary Michael Gove also on Sunday sparked questions over the charity worker's fate by appearing uncertain over why she was in Iran.
Asked what Nazanin Zaghari-Ratcliffe was doing in Tehran when she was arrested and detained in 2016, he admitted: "I don't know."