Speaking today on Good Morning Britain, Richard Ratcliffe said: 'Nazanin was pretty low in the build up to this but this has really lifted her - just the fact that the Foreign Secretary has so robustly defended her'.
But Iran quickly stated the London decision was a violation of worldwide law.
However, Iran refuses to recognise dual nationals so does not recognise Mrs Zaghari-Ratcliffe's right to be represented by Britain.
IRAN has accused Britain of breaking worldwide law by taking the rare step of granting diplomatic protection to a dual British-Iranian mother jailed in Teheran over spy allegations.
It is unlikely the United Kingdom will try to use legal levers as part of efforts to secure Mrs Zaghari-Ratcliffe's freedom, as they would be lengthy and she is already nearly three years into a five-year sentence.
Mr Hunt said his decision was "an important diplomatic step" signalling to Tehran that "its behaviour is totally wrong".
Next month will mark three years since Nazanin Zaghari-Ratcliffe was taken from her family and denied her freedom.
The granting of diplomatic protection will have no immediate impact on Mrs Zaghari-Ratcliffe's conditions in jail in Tehran.
Prime Minister Theresa May has also called for her release, directly appealing to Iranian President Hassan Rouhani to free the mother at the UN's general assembly in NY past year.More news: Akash-Shloka wedding: Antilia decked up with flowers
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He added: "Iran was an ancient civilisation, but no government should use innocent individuals as pawns for diplomatic leverage".
Zaghari-Ratcliffe's daughter, now four, has also been unable to leave Iran since her mother's detainment so that she can visit twice per week.
She was subsequently sentenced to five years imprisonment, despite strenuously denying the charges against her, while Gabriella remains in the country cared for by family.
Iranian officials insist she was arrested for spying and trying to overthrow the Tehran government. "So she's obviously not in a great position". It will hopefully be the first step towards finally ensuring that Nazanin is freed, and that she and her daughter are reunited with their husband and father in the UK.
It is an official recognition by the British government that Iran had not met its obligations under global rules in regard to her treatment and makes it a formal matter between the states.
The U.K. government has made a decision to provide diplomatic protection to a British-Iranian woman detained in Iran for almost three years.
While it does not automatically dictate any particular course of action, the Foreign Office has indicated it will take the steps it believes are most likely to secure the goal of Mrs Zaghari-Ratcliffe's release.
Hunt said London wanted to make Iran aware that it had no intention of letting Zaghari-Ratcliffe languish in jail.