Britain is placing Nazanin Zaghari-Ratcliffe – the UK citizen whose jailing in Iran has become an issue of huge international controversy – under diplomatic protection, in a move expected to have a significant impact in efforts to get her freed.

The decision raises Ms Zaghari-Ratcliffe’s incarceration from a consular to a state-to-state issue and will, says foreign secretary Jeremy Hunt, oblige “Iran to fulfil its obligations under international law”. It also opens the door for legal action with the possibility of compensation if it is proved that she had been mistreated.

On a broader basis, British diplomats making representations to Tehran on behalf of Ms Zaghari-Ratcliffe, who has dual British and Iranian nationality, will no longer be doing so on behalf of just an individual, but will be acting on behalf of the British state.

The offer of diplomatic protection is a “very rare move”, with the Foreign and Commonwealth Office in London saying “the UK has not used it in recent memory”, according to an official statement.

In offering Ms Zaghari-Ratcliffe this protection, the British government holds that the legal action brought against Ms Zaghari-Ratcliffe in Iran “failed to meet Iran’s obligations under international standards” and that she “has periodically been denied access to the medical care recommended by doctors” despite assurances by the Iranian government.

Nazanin Zaghari-Ratcliffe – in pictures Show all 20 1 /20 Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe – in pictures 2018 Nazanin Zaghari-Ratcliffe hugs her daughter Gabriella, in Iran after she was allowed to leave the Iranian prison, she is being held in, for three days. Zaghari-Ratcliffe was arrested during a holiday with her toddler daughter in April 2016. Iranian authorities accuse her of plotting against the government. Her family denies this, saying says she was in Iran to visit family. Free Nazanin Campaign/AP Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her husband Richard Ratcliffe and their daughter Gabriella. Nazanin is serving a five-year prison sentence for allegedly plotting to overthrow Iran's government. PA Nazanin Zaghari-Ratcliffe – in pictures June 2016 Richard Ratcliffe's daughter Gabriella had her British passport confiscated and was stranded in Iran with her grandparents after her mother Nazanin was jailed. He left left a giant birthday card on the doorstep of the Iranian embassy in central London to mark her second birthday in June 2016. PA Nazanin Zaghari-Ratcliffe – in pictures Nazanin has spent some of her prison sentence in solitary confinement. PA Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her husband Richard and daughter Gabriella. Family Handout Nazanin Zaghari-Ratcliffe – in pictures July 2016 Richard Ratcliffe delivering a letter of petition with his mother Barbara Ratcliffe and MP Tulip Siddiq, to 10, Downing Street on the 100th day of her detention, on July 12, 2016. Getty Nazanin Zaghari-Ratcliffe – in pictures Supporters of Nazanin Zaghari-Ratcliffe held a vigil outside the Foreign and Commonwealth Office to mark her 707 days in captivity. Getty Nazanin Zaghari-Ratcliffe – in pictures January 2017 Richard Ratcliffe holds a '#Free Nazanin' sign and candle during a vigil for for wife on January 16, 2017. The vigil, being held outside the Iranian Embassy in London marks one year since the Washington Post journalist Jason Rezaian and other US-Iranian dual-nationals were released from prison in Iran. Getty Nazanin Zaghari-Ratcliffe – in pictures Nazanin with her daughter Gabriella before they were detained by Iranian authorities. Change.org Nazanin Zaghari-Ratcliffe – in pictures September 2017 Gabriella, who is three-years-old in this picture, has now spent two years away from her mother. Richard Ratcliffe Nazanin Zaghari-Ratcliffe – in pictures November 2017 Foreign Secretary Boris Johnson meets with Richard Ratcliffe over Nazanin's case. They meet just days after Johnson told a parliamentary committee that she was in Iran "training journalists". WPA Pool/Getty Nazanin Zaghari-Ratcliffe – in pictures November 2017 Actor Emma Thompson braved pneumonia to support Richard Ratcliffe in leading demonstrators before a march in support of Nazanin in November. Reuters Nazanin Zaghari-Ratcliffe – in pictures November 2017 Richard Ratcliffe after the march said: 'It is profoundly moving to see so many people here.' REUTERS Nazanin Zaghari-Ratcliffe – in pictures November 2017 A picture of Nazanin Zaghari-Ratcliffe shown on Iranian state TV as part of a report that made fresh allegations against her. They said she had been recruiting for banned broadcast services, as well as 'opposition cyber teams'. Nazanin Zaghari-Ratcliffe – in pictures December 2017 Iranian president Hassan Rouhani greets British Foreign Secretary Boris Johnson at the presidential office in Tehran, Iran. Johnson visited Tehran to discuss the fate of detained Nazanin Zaghari-Ratcliffe. EPA Nazanin Zaghari-Ratcliffe – in pictures Nazanin Zaghari-Ratcliffe with her daughter Gabriella. PA Nazanin Zaghari-Ratcliffe – in pictures December 2017 Photos of Richard Ratcliffe and his wife Nazanin Zaghari-Ratcliffe on display at their home in north London. Mr Ratcliffe said he believed there was "still a chance" she may be released from an Iranian prison in time for a dream Christmas together. Unfortunately that didn't happen. PA Nazanin Zaghari-Ratcliffe – in pictures February 2018 Richard Ratcliffe delivers a petition and a letter addressed to the Iranian Deputy Foreign Minister Abbas Araghchi to demand her release, at the Iranian Embassy in London on February 21, 2018. He also left support letters for his spouse in the country's embassy, amid a visit by a deputy foreign minister. AFP/Getty Nazanin Zaghari-Ratcliffe – in pictures August 2018 Foreign Secretary, Jeremy Hunt meeting Richard Ratcliffe. Hunt has pledged to do everything possible to secure the release of a charity worker jailed in Iran Jeremy Hunt/PA Nazanin Zaghari-Ratcliffe – in pictures August 2018 Nazanin Zaghari-Ratcliffe hugs her daughter Gabriella, in Iran after she was allowed to leave the Iranian prison, she is being held in, for three days. Zaghari-Ratcliffe was arrested during a holiday with her toddler daughter in April 2016. Iranian authorities accuse her of plotting against the government. Her family denies this, saying says she was in Iran to visit family. PA

Iranian officials insist that she is receiving the medical treatment necessary and that she has not suffered any abuse.

Ms Zaghari-Ratcliffe’s husband Richard, who has been campaigning in Britain for her release, has been told about the step being taken by the government. The couple have a three-year-old daughter being cared for by Ms Zaghari-Ratcliffe’s family in Tehran.

Mr Ratcliffe had in the past asked Boris Johnson, when he was foreign secretary, to declare diplomatic protection for his wife.

Any subsequent legal action taken would have to go through the International Court of Justice in The Hague. Senior diplomatic officials stressed, however, that no decision has been made about taking that path at this point.

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Iran does not accept dual nationality and Tehran has repeatedly stated that Ms Zaghari-Ratcliffe is Iranian. Thus, they hold, she cannot be treated as a UK citizen, and the British government has no right to interfere with the country’s internal judicial process.

Mr Hunt acknowledged that diplomatic protection is “unlikely to be a magic wand that leads to an overnight result”, but “it demonstrates to the whole world that Nazanin is innocent and the UK will not stand by when one of its citizens is treated so unjustly”.

British government officials maintain that there is no connection between her case and that of Iran’s nuclear agreement with international powers – which Donald Trump is trying to tear up – but which Britain, along with France, Germany, Russia and China, the other signatories to the deal, is trying to preserve.

But it is hoped that France and Germany, along with other western states and the United Nations, will back the UK in taking robust steps to get Ms Zaghari-Ratcliffe freed.

Last October, Jose Antonio Guevara Bermdez, chair-rapporteur of the UN’s Working Group on Arbitrary Detention, and Asma Jahangir, special rapporteur on human rights for Iran, called for Ms Zaghari-Ratcliffe’s immediate release.

Ms Zaghari-Ratcliffe has been detained in Iran since 3 April 2016. In September that year she was sentenced to five years’ imprisonment for “plotting to topple the Iranian government”.

The prosecutor general in Tehran said that she was being held for running “a BBC Persian online journalism course which was aimed at recruiting and training people to spread propaganda against Iran”.

Ms Zaghari-Ratcliffe and her family vehemently deny the allegations.

Speaking about the exercise of diplomatic protection, Mr Hunt said that: "Nazanin Zaghari-Ratcliffe is an innocent woman who has spent the last three years in an Iranian jail, separated from her daughter and husband. We have been working hard to secure her release but despite repeated efforts have not been successful. We have not even been able to secure her the medical treatment she urgently needs despite assurances to the contrary.

“So I have today decided that the UK will take a step that is extremely unusual and exercise diplomatic protection. I have not taken this decision lightly. I have considered the unacceptable treatment Nazanin has received over three years, including not just lack of access to medical treatment but also lack of due process in the proceedings brought against her.