Hunt said that he made a decision to take the "extremely unusual" step of granting a citizen diplomatic protection because of the "unacceptable treatment" of Zaghari-Ratcliffe, who he called "an innocent woman".
He added, "Govts may only exercise such protection for own nationals".
The British Foreign and Commonwealth Office advises on its website there is "a risk that British nationals, and a higher risk that British/Iranian dual nationals, could be arbitrarily detained in Iran".
"Ms Zaghari thus remains Iranian", he said.
IRAN has accused Britain of breaking global law by taking the rare step of granting diplomatic protection to a dual British-Iranian mother jailed in Teheran over spy allegations.
"We have not even been able to secure her the medical treatment she urgently needs despite assurances to the contrary", Hunt added.
Hunt told the BBC late Thursday night that the move was unlikely to be a "magic wand" to get her released, but was an "important diplomatic step".
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In January, Mrs Zaghari-Ratcliffe went on hunger strike for three days saying the Iranian prison authorities had refused her treatment for a series of medical conditions including lumps in her breasts, severe neck pain, and numbness in her arms and legs.
A photo of Zaghari-Ratcliffe is seen amongst candles during a fourth birthday vigil for her daughter Gabriella opposite the Foreign and Commonwealth Office, on June 11, 2018. Hunt also raised the issue with the Iranian foreign minister, Javed Zarif, on a visit to Tehran in December, but the FCO says its officials have been unable to access those in the revolutionary guard ultimately responsible for her fate.
"It sends a very strong message to Iran: You are a great civilisation, you may have disagreements with the United Kingdom, but at the heart of this is an innocent woman, vulnerable, unwell and scared", Hunt said. Diplomatic protection has now elevated it to a formal inter-state dispute between Britain and Iran.
Zaghari-Ratcliffe has suffered from a range of health issues since being arrested while leaving Tehran after taking her infant daughter on a family visit in April 2016. Not only this, but it gives Britain more power take worldwide legal action (though Hunt indicated that this was unlikely) and also to address the maltreatment of Zaghari-Ratcliffe in other global powerhouses like the United Nations.
Last August, the mother was unexpectedly released for a three-day furlough and was reunited with members of her family including her daughter Gabriella outside the Iranian capital.
Hunt said London wanted to make Iran aware that it had no intention of letting Zaghari-Ratcliffe languish in jail. Her five-year prison sentence for plotting the "soft toppling" of its government has been widely criticized.