Boris Johnson has come under renewed fire over his handling of the Nazanin Zaghari-Ratcliffe case after a senior official accused him of trying to "protect himself" rather than focusing on "getting this poor woman home".
Dave Penman, general secretary of the civil servants' union FDA, issued a stinging rebuke to the Foreign Secretary over reports suggesting it was Foreign Office officials who were responsible for Mr Johnson's mistakenly saying the jailed mother had been training journalists in Iran.
"It's difficult to imagine such a mistake being made by an FCO official but, even if it were, the buck stops with the Foreign Secretary," said Mr Penman.
"He should be focusing on getting this poor woman home to her family, not looking to protect himself with anonymous briefings to journalists.
"It's very disappointing that these accusations are being thrown around, safe in the knowledge that civil servants are unable to defend themselves or respond."
Mr Johnson's gaffe has prompted fears that the 38-year-old charity administrator is facing an additional five years in jail and prompted calls for the Foreign Secretary's resignation.
Allies of Mr Johnson have reportedly said he was given an incorrect briefing by Foreign Office officials ahead of a committee hearing at which the comments were made.
The Iranian judiciary and media seized upon Mr Johnson's statement as proof of her guilt and she is now facing a new trial for "propaganda against the regime", which could see her face an additional time in jail.
Sources close to the Foreign Secretary have told Sky News it is "totally untrue"that Mr Johnson is seeking to blame Foreign Office officials, or their briefing, for his comments.
But the stinging criticism from Mr Penman piles further pressure on the Foreign Secretary, who has faced calls for his resignation in recent days from across the political spectrum.
Tory backbencher Anna Soubry said "in 'normal' times Boris Johnson would have been sacked long ago".
Mrs Zaghari-Ratcliffe's family has also stepped up the pressure in recent days as the mother-of-one's plight hit the national airwaves.
Her husband Richard told Sky News he was now in contact with senior politicians over the case.
Foreign Office minister Alistair Burt told Mr Ratcliffe on Friday "serious consideration" was being given to allowing him to travel to Iran with Mr Johnson.
The pair are due to have a conversation on Sunday – the first direct contact between the two since Mrs Zaghari-Ratcliffe was first incarcerated in April 2016 while on holiday in Iran visiting her family with her two-year-old daughter Gabriella.
She was separated from her daughter and spent the first eight months in solitary confinement as she was interrogated by Revolutionary Guards.
Mr Johnson has apologised for causing any anxiety by mistakenly telling MPs that Mrs Zaghari-Ratcliffe had been training journalists at the time she was arrested in Tehran.