The husband of a British-Iranian woman, in prison in Iran, has urged Boris Johnson to retract his remark that she had been training journalists in Iran.
Richard Ratcliffe said Iranian officials were using the foreign secretary's statement to justify extending his wife Nazanin's jail term.
Mr Johnson has called his Iranian counterpart to clarify the situation.
Liam Fox said his cabinet colleague's comment was a "slip of the tongue" but Labour said it was "unacceptable".
Mrs Zaghari-Ratcliffe is serving a five-year sentence for allegedly plotting to topple the government in Tehran, although the official charges were never made public.
Mr Ratcliffe maintains his wife's innocence, saying she was in Iran visiting family when she was arrested in 2016.
She was summoned to court on Saturday where Mr Johnson's comment was cited as new evidence as to what she was doing in Iran.
The Foreign Office said Mr Johnson now accepted he "could have been clearer" with his comments to the Foreign Affairs Committee.
In a phone call to Iranian foreign minister Javad Zarif, Mr Johnson said that his remarks provided "no justifiable basis" for further legal action and he intended to visit Iran before the end of the year to discuss the case.
Mr Zarif told the foreign secretary that the developments in the case over the weekend "were unrelated" to Mr Johnson's remarks, a Foreign Office statement added.
International Trade Secretary Liam Fox told BBC Radio 4's Today programme: "We all make slips of the tongue", adding on Sky News that it wasn't "a serious gaffe".
But Labour's Tulip Siddiq, Mrs Zaghari-Ratcliffe's MP, said it was "unacceptable" for Mr Johnson to "lack a basic grasp on this important situation".
"A slip of the tongue in my books is swearing in front of your child by mistake - it's not condemning a British citizen to an extra five years in prison," she said.
Mr Ratcliffe told the BBC's Today programme that Mr Johnson should make a statement in the Commons, retracting what he said.
He added the "worst thing" Mr Johnson could do now was "suddenly go quiet and to create this problem without making any clarifications".
He said he still hoped his wife and young daughter would be back home in the UK for Christmas.
Former Conservative foreign secretary Sir Malcolm Rifkind told the BBC Mr Johnson needed "to concentrate more" and "get the detail right".
Mrs Zaghari-Ratcliffe was sentenced to jail following a court hearing into whether she was attempting to overthrow the government.
She denies all the allegations against her, but lost her final appeal in April.
She has since faced two more charges relating to an accusation of plotting to topple the regime in Tehran.
Mrs Zaghari-Ratcliffe has worked for the Thomson Reuters Foundation and the BBC, but insisted the 2016 visit was for her daughter to meet her grandparents.
However, appearing before MPs on the House of Commons Foreign Affairs Select Committee last week, Mr Johnson appeared to contradict that.
He criticised Iran over the case before saying: "When I look at what Nazanin Zaghari-Ratcliffe was doing, she was simply teaching people journalism, as I understand it.
"[Neither] Nazanin Zaghari-Ratcliffe nor her family has been informed about what crime she has actually committed. And that I find extraordinary, incredible."
Four days later, Mrs Zaghari-Ratcliffe was summoned to court where the foreign secretary's comments were cited as fresh evidence against her.
At the hearing, she was accused of engaging in "propaganda against the regime".
The Iranian judiciary's High Council for Human Rights said Mr Johnson's comments "shed new light" on the charity worker and proved Mrs Zaghari-Ratcliffe "had visited the country for anything but a holiday".
Mrs Zaghari-Ratcliffe had been eligible for parole under the early release scheme from 23 November.
However, Mr Ratcliffe told the Press Association that she could now face a fresh trial before that date to block her chance of freedom.
"I think the one thing the foreign secretary could do to make amends would be if he went to visit her in the next few weeks before her trial," he said.
"Careless talk has a cost and there's been a lot of careless talk."
Monique Villa, chief executive of the Thomson Reuters Foundation, called on Mr Johnson to "immediately correct the serious mistake he made".