UK Foreign Secretary Boris Johnsons has come under fire after a "mistake" led to imprisoned Iranian-British aid worker Nazanin Zaghari-Ratcliffe potentially facing an extra five year sentence in Iran.

Johnson told the British parliament's foreign affairs committee on 1 November that Zaghari-Ratcliffe - who is already serving a five-year jail sentence after being convicted of plotting to overthrow Iran's clerical establishment - had been teaching people journalism, a statement that the Thomson Reuters Foundation, a charity organisation for which she works, said was incorrect.

“If you look at what Nazanin Zaghari-Ratcliffe was doing, she was simply teaching people journalism as I understand it, at the very limit," Johnson told the committee.

His comments led to Zaghari-Ratcliffe again being brought into court on 4 November, three days after Johnson's remarks, and accused by a judge of "spreading propaganda against the regime".

The charges against Nazanin were denied by her family and the foundation.

An undated handout image released by the Free Nazanin campaign in London on June 10, 2016 shows Nazanin Zaghari-Ratcliffe (R) posing for a photograph with her husband Richard and daughter Gabriella (AFP)

"She is not a journalist and has never trained journalists at the Thomson Reuters Foundation where she is project manager in my media development team," Monique Villa, Thomson Reuters Foundation CEO, said in a statement.

On Monday, Britain said the comments made by Johnson about Zaghari-Ratcliffe should not be used to bring additional charges against her.

"Last week's remarks by the foreign secretary provide no justifiable basis on which to bring any additional charges against Nazanin Zaghari-Ratcliffe," a spokesman for the British Foreign Office said.

"While criticising the Iranian case against Mrs Zaghari-Ratcliffe, the foreign secretary sought to explain that even the most extreme set of unproven Iranian allegations against her were insufficient reason for her detention and treatment."

Johnson's comments provoked outrage among British politicians and commentators, some of whom called for the minister to be sacked.

"We've all become accustomed to Boris Johnson's foot-in-mouth syndrome, but as foreign secretary his words matter," said Labour MP Wes Streeting, speaking to the Sun.

"This isn't his first gaffe in recent months, but it looks as though his careless attitude will result in a longer prison sentence for an innocent British national.

"This is intolerable. He should be sacked."

Zaghari-Ratcliffe used to work in London for the BBC, which Iran says is seeking to topple the Islamic theocracy.

Iran refuses to recognise dual nationals and denies them access to consular assistance.