Michael Gove has come under fire after appearing to cast doubt over the actions of a British woman jailed in Iran.
Labour accused the Environment Secretary of bungling after saying he did not know what Nazanin Zaghari-Ratcliffe was doing in the country.
Mr Gove mounted a staunch defence of Boris Johnson, who has been criticised for making remarks that were seized on by Tehran to justify its threats to extend her sentence.
The country's state TV broadcaster claimed the Foreign Secretary's suggestion Mrs Zaghari-Ratcliffe was "training journalists" in Iran at the time of her arrest last year amounted to an "unintended admission" of her guilt.
Asked what she was doing in Iran, Mr Gove told BBC One's The Andrew Marr Show: "I don't know.
"One of the things I want to stress is there is no reason why Nazanin Zaghari-Ratcliffe should be in prison in Iran so far as any of us know."
Mr Gove warned it would be a "big mistake" to focus attention on his Cabinet colleague when Iran is "in the dock".
"There is an effort, somehow, to shift attention and direction away from who is really at fault her and it is the Iranian regime," he said.
Mr Gove said that there is no reason for Mrs Zaghari-Ratcliffe's detention and she should be released.
"There is nothing the Iranian regime would like more than for the attention to be shifted off them and on to us", Mr Gove said.
"I think we make a big mistake if we think the right thing to do is to blame politicians in a democracy who are trying to do the right thing for the plight of a women who has been imprisoned by a regime that is a serial abuser of human rights.
Shadow Cabinet office minister Jon Trickett said Theresa May must ensure Nazanin Zaghari-Ratcliffe "does not pay the price for her ministers' bungling".
"Boris Johnson's cavalier approach to international diplomacy is compounded this morning by Michael Gove claiming he has no idea what Nazanin Zaghari-Ratcliffe was doing in Iran. It has always been clear, she was on holiday visiting her family," he said.
"It appears Gove is more interested in protecting Johnson's job than the liberty of a British citizen in jail in Iran.
Labour former minister Chris Bryant said: "It is scandalous that Gove adds further ambiguity to Nazanin's position by being so poorly briefed. Look, ministers, the Iranians are dastardly so you need to be ruthlessly self-disciplined or you put Britons at risk."
Labour's Sadiq Khan said Mr Johnson should resign or be sacked following his "long list of gaffes" as Foreign Secretary.
The London mayor raised questions about Mr Johnson's suitability for the role after renewing calls for an apology and clarification over the Tory MP's remarks, which were made during an appearance before a Commons committee.
Mrs Zaghari-Ratcliffe and her employer, Thomson Reuters Foundation, insisted the training claim is incorrect with Mr Johnson later acknowledging his comments "could have been clearer" and he had no doubt "she was on holiday" in Iran.
Mr Khan, who succeeded Mr Johnson as London mayor, told Marr: "I think he's got to go. I think he's our Foreign Secretary whose job is diplomacy and representing the best interests of our country and if Theresa May was a strong Prime Minister she would have sacked him a long time ago."
Mrs Zaghari-Ratcliffe was arrested as she tried to leave Iran following a family holiday with her two-year-old daughter in April 2016 and is accused of plotting to topple the Iranian regime.