Following criticism for refusing to act, Mr Johnson has finally attacked both Nazanin's conviction for spying as a mockery of justice and her treatment by the Iranian authorities.
But Labour withdrew its demands for him to quit after her husband warned this would not help her case.
Richard Ratcliffe hit out this week after Iran reacted to Mr Johnson wrongly claiming Nazanin was "training journalists" by hauling her back to court and threatening to increase her jail term to five years.
Johnson said on November 1 that she had been teaching people journalism before her arrest in April 2016, contradicting her and her employer, who said she had been on holiday visiting her family. She has been confined in a high-security Iranian jail ever since while her daughter, now three, remains at the Tehran home of Nazanin's parents. He is likely to accompany the foreign secretary on his imminent visit to Tehran so he can see his wife, who has been held largely in solitary confinement since April 2016. The mother-of-one has a family history of breast cancer and a former cellmate described seeing her hair falling out in "huge clumps".
"But, as it turns out, they have not paid attention to the fact that the principle of political protection is applicable to the citizens of the protecting country; thus, in as much as, Zaghari has dual British-Iranian citizenship, and Iran doesn't recognise her British citizenship, the principle is fundamentally impractical".
Ratcliffe will meet Johnson and his consular specialists at the Foreign Office on Wednesday.
Her husband has campaigned for her release and has insisted she was on holiday with their 18-month-old daughter Gabriella so the child could meet her grandparents in Iran.
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There are concerns within the Foreign Office that Ratcliffe might still be arrested by the Iranian police, even if he is accompanying Johnson.
A Foreign Office statement to the BBC said Johnson and Ratcliffe will meet to discuss a request to give Zaghari-Ratcliffe "diplomatic protection" - which, under worldwide law, allows a state to take diplomatic action on behalf of a national.
The meeting comes as Mr Johnson faces calls for his resignation after he suggested to MPs that Mr Ratcliffe's wife, an aid worker, was in Iran to train journalists.
Mr Ratcliffe added: "My wife is angry now". She loses her temper over the smallest things. "As Foreign Secretary, I would like you to instruct your department personally to give her that protection".
A statement from the Foreign Office said that Mr Johnson had "made it clear that no stone should be left unturned" in securing the release of Mrs Zaghari-Ratcliffe.
The family's local MP Tulip Siddiq told Sky News that Mrs Zaghari-Ratcliffe had sobbed inconsolably during an earlier phone call with her husband, saying: 'She just said that she couldn't believe that Boris Johnson would make those comments.