The Foreign Office (FCO) has escalated its conflict with Iran over the imprisonment of Nazanin Zaghari-Ratcliffe by granting her diplomatic protection, a step that raises her case from a consular matter to the level of a dispute between the two states.

The move, likely to lead to increased tensions between the two countries, came after the foreign secretary, Jeremy Hunt, judged Tehran was not meeting its human rights obligations to the British-Iranian dual national under international humanitarian law. She has been in a Tehran jail for three years, after being sentenced to five years for spying.

Hunt toughened his stance amid claims that the Iranians ha.d spurned her request for better medical treatment, including regular access to drugs and to her own doctor. Zaghari-Ratcliffe has been in jail since April 2016, when she was arrested at Tehran airport following a visit to her family with her then 22-months-old daughter.

Affording Zaghari-Ratcliffe diplomatic protection, a step advocated by campaign group Redress and her husband Richard for nearly two years, also means that from now on, an injury to her is viewed as an injury to the British state.

“We have not been able to secure her the medical treatment she urgently needs despite the assurances to the contrary,” Hunt said.

Q&A What is diplomatic protection? Show Granting diplomatic protection is a rarely used diplomatic device which elevates a dispute from being a consular matter to being a formal state-to-state issue. Under international law, it is a way for a state to take action on behalf of a national whose rights have been breached by another country. In this case, the foreign secretary, Jeremy Hunt, said diplomatic protection was invoked in recognition of Iran's failure to meet its obligations under international law in its treatment of Nazanin Zaghari-Ratcliffe. Diplomatic protection can take the form of consular action, political and economic pressure, negotiations with the other state, judicial proceedings or other forms of peaceful dispute settlement. It is distinct from diplomatic immunity, which covers the status of accredited diplomats. Photograph: HANDOUT/AFP

Admitting the step was “very unusual”, Hunt added: “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 proceedings brought against her”.

He said the step was a signal that “Tehran’s behaviour is totally wrong”.

He admitted “the measure is unlikely to be a magic wand that leads to an overnight result. But it demonstrates that Britain will not stand by when one of its citizens is treated so unjustly”.

He added: “Iran was an ancient civilisation, but no government should use innocent individuals as pawns for diplomatic leverage.”

The practical implication of such an elevation depends largely on how the FCO chooses to respond, but it could mean the case is raised on the international stage and in extremis, could lead to sanctions, and requests for reparations on the basis that Britain is the victim of an unlawful act.

Nazanin Zaghari-Ratcliffe embracing her daughter Gabriella following her release from prison for three days in August 2018. Photograph: -/AFP/Getty Images

Richard Ratcliffe said believed his wife would welcome the decision. “I last spoke to her on Wednesday and obviously I knew that the Foreign Secretary had asked did we still want diplomatic protection, so she was a bit more upbeat than she’d been before,” he told the BBC Radio 4 Today programme. “She got very low again recently. I’m sure this will give her a big lift.”

Suggesting the move might provide “a route to solving this case”, Ratcliffe said: “Probably the first things are to get a doctor in, then we’ve talked about doing stuff at the UN, then maybe summoning the ambassador.

“You can’t jump straight into legal action, there are a whole series of things that legally you need to do first before you are entitled to, and that’s months away. Hopefully we are not months away from a solution.”

Timeline Imprisonment of Nazanin Zaghari-Ratcliffe in Iran Show Arrest in Tehran Nazanin Zaghari-Ratcliffe is arrested at Imam Khomeini airport as she is trying to return to Britain after a holiday visiting family with her daughter, Gabriella. Release campaign begins Her husband, Richard Radcliffe, delivers a letter to David Cameron in 10 Downing Street, demanding the government do more for her release. Sentenced She is sentenced to five years in jail. Her husband says the exact charges are still being kept a secret. Hunger strike Nazanin Zaghari-Ratcliffe's health deteriorates after she spends several days on hunger strike in protest at her imprisonment. Appeal fails Iran’s supreme court upholds her conviction. Boris Johnson intervenes Boris Johnson, then Foreign Secretary, tells a parliamentary select committee "When we look at what [she] was doing, she was simply teaching people journalism". Four days after his comments, Zaghari-Ratcliffe is returned to court, where his statement is cited in evidence against her. Her employers, the Thomson Reuters Foundation, deny that she has ever trained journalists, and her family maintain she was in Iran on holiday. Johnson is eventually forced to apologise for the "distress and anguish" his comments cause the family. Health concerns Her husband reveals that Zaghari-Ratcliffe has fears for her health after lumps had been found in her breasts that required an ultrasound scan, and that she was now “on the verge of a nervous breakdown”. Hunt meets husband New Foreign Secretary Jeremy Hunt meets with Richard Ratcliffe, and pledges "We will do everything we can to bring her home." Temporary release She is granted a temporary three-day release from prison. Hunger strike Zaghari-Ratcliffe is on hunger strike again, in protest at the withdrawal of her medical care. Diplomatic protection The foreign secretary, Jeremy Hunt, takes the unusual step of granting her diplomatic protection – a move that raises her case from a consular matter to the level of a dispute between the two states. Travel warning The UK upgrades its travel advice to British-Iranian dual nationals, for the first time advising against all travel to Iran. The advice also urges Iranian nationals living in the UK to exercise caution if they decide to travel to Iran. Hunger strike in London Richard Ratcliffe joins his wife in a new hunger strike campaign. He fasts outside the Iranian embassy in London as she begins a third hunger strike protest in prison. Hunger strike ends Zaghari-Ratcliffe ends her hunger strike by eating some breakfast. Her husband also ends his strike outside the embassy.

Moved to mental health ward According to her husband, Zaghari-Ratcliffe was moved from Evin prison to the mental ward of Imam Khomeini hospital, where Iran’s Revolutionary Guards have prevented relatives from contacting her. Daughter returns to London Zaghari-Ratcliffe's five year old daughter Gabriella, who has lived with her grandparents in Tehran and regularly visited her mother in jail over the last three years, returns to London in order to start school. Temporary release Amid the threat of the coronavirus pandemic, she is temporarily released from prison, but will be required to wear an ankle brace and not move more than 300 metres from her parents’ home. New charges Iranian state media reports that she will appear in court to face new and unspecified charges. In the end, a weekend court appearance on a new charge of waging propaganda against the state that could leave her incarcerated for another 10 years is postponed without warning, leading Zaghari-Ratcliffe to say "People should not underestimate the level of stress. People tell me to calm down. You don’t understand what it is like. Nothing is calm."

Their four-year-old daughter, Gabriella, remains with Zaghari-Ratcliffe’s mother in Tehran.

The FCO has been resisting campaigners’ calls for an escalation for more than two years, partly because it is a rare step to take on an individual consular case and partly due to fears it may have reverberations on wider British-Iranian relations.

The UK has recommitted itself to the Iran nuclear deal, but Tehran is frustrated that Europe has not done more to ease trade with Iran.

An Iranian-British dual national, Zaghari-Ratcliffe went on hunger strike for three days in January this year in an attempt to improve her treatment, but this week she wrote to Iranian authorities to claim promises of improved treatment have not materialised.

In a letter to Tehran’s prosecutor, Abbas Jafari Dolatabadi, she complained she was “still being deprived of medical attention and from being sent to [outside] medical facilities. During the years that we have spent in prison, sending us to medical centres to [see] our doctors was a usual occurrence but, for unknown reasons, it is no longer done.

“Access to doctors, treatment and medication is the right of any human being and prisoners are no exception.”

She said repeated requests for better treatment have been ignored, adding that if anything happened to her or if her illnesses get worse, the responsibility would lie with the officials of the Islamic Republic.

She wrote the letter in a joint plea with another Iranian prisoner in Evin prison, Narges Mohammadi.

The FCO has been resisting campaigners’ calls for an escalation for more than two years, partly because it is a rare step to take on an individual consular case and partly due to fears it may have reverberations on wider British-Iranian relations.

The UK has recommitted itself to the Iran nuclear deal, but Tehran is frustrated that Europe has not done more to ease trade with Iran.

Iran does not recognise dual national status, and so do not formally recognise Zaghari-Ratcliffe’s right to be represented by Britain. Iranian officials insist she was arrested for spying and trying to overthrow the Tehran government.

Hunt 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.

Tulip Siddiq, the constituency MP for the Ratcliffe family, said: “For three years we have fought for the government to treat Nazanin as a British citizen in this dispute and now it is clear that they will. Iran must now do the right thing and free my constituent from her prison cell.

“Iran has violated numerous international human rights standards, including solitary confinement, no fair trial, denial of medical care and has threatened the Ratcliffe family.”