Nazanin Zaghari-Ratcliffe, a British charity worker jailed in Iran, has announced she will start a hunger strike on 14 January after being refused access to urgently needed medical care.

Writing from Tehran’s Evin prison, Ms Zaghari-Ratcliffe said she will initially start a three-day strike with fellow inmate and prominent rights activist Narges Mohammadi that will continue until their demands are met.

The British dual national and mother-of-one has been behind bars since she was arrested from Tehran airport while on holiday visiting family in April 2016.

Her husband Richard Ratcliffe told The Independent that prison officials had refused to allow doctors to examine lumps in her breasts or to proceed with neurological care over her neck pains and numbness in her arms and legs.

He said inmates were also being threatened with reduced phone calls and restricted access to food and has called for an urgent meeting with the Iranian ambassador to London.

The joint letter, published by Tehran-based Defenders of Human Rights Centre (DHRC) on Thursday, said both women had been barred access to medication and treatment “despite frequent requests” and appeals to relevant authorities. Emergency referral to outside clinics by Evin’s own prison doctor was also ignored by the authorities.

“In protest against this illegal, inhuman and unlawful behaviour, and to express our concerns for our health and survival at this denial of specialist treatment, despite taking daily medicines, we will go on hunger strike from 14 to 16 January and ask for immediate care,” the letter read.

“In the event of the authorities’ failure to address these concerns and them further endangering our health, we will take further actions. The authorities of the Islamic Republic of Iran are to be held responsible for the potential consequences,” it added.

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

Ms Zaghari-Ratcliffe, who turned 40 last week in jail, was separated from her 21-month daughter at Tehran airport and taken into detention in April 2016 while she was attempting to return home to London. She was left at risk of suicide after eight months in solitary confinement.

She was later handed a five-year jail sentence on espionage charges she denies, in a short trial where she was barred proper access to legal representation.

Her incarceration has been widely condemned as “a travesty of justice”. The Islamic Republic has so far denied appeals from senior British officials to release her.

Her 1,000th day behind bars was marked on 28 December.

Mr Ratcliffe, who spoke to his wife on Sunday, told The Independent the family were seriously concerned about the hunger strike, fearing that she is not physically strong enough to pull through it.

[They have been] denied healthcare but there are also threats of reduced calls and restrictions on families buying extra food Richard Ratcliffe, husband to Nazanin Zaghari-Ratcliffe jailed in Iran

“I had not wanted her to take this step. But she and Narges felt it was time, as there have been a number of pressures in the ward just recently – [they have been] denied healthcare but there are also threats of reduced calls and restrictions on families buying extra food,” he explained.

“I think it could be a really tough January for us – we are still half hoping that something moves so that it doesn’t happen,” he added.

On Wednesday Mr Ratcliffe also wrote to Iran’s foreign minister Mohammad Javad Zarif to request an urgent meeting with Iran’s ambassador to London “saying things are escalating too far for them to continue never to meet”.

Ms Mohammadi, 46, who is one of Iran’s best-known activists and vice president of the DHRC, has been incarcerated since 2015, when she was arrested and charged with “collusion against national security” and “committing propaganda against the state”. She is currently serving a 16-year prison sentence.

Her brother Hamidreza, 40, told The Independent Ms Mohammadi first fell ill after a long stint in solitary confinement. Like Nazanin, she has a neurological disorder which causes muscular paralysis that the family fear could become permanent. She also has an untreated and unexamined growth in her stomach.

They know the dangers of the hunger strike, it is their last resort Hamidreza, brother of hunger-striking activist Nagres Mohammadi

“She has serious problems and without access to medication it will endanger her health and life,” he said.

“They know the dangers of the hunger strike, it is their last resort. If the authorities don’t grant them proper medical attention, it is very likely they will keep hunger striking,” he added.

International rights groups urged the Iranian authorities to allow the women immediate access to medical care and called on UK government officials to put pressure on their Iranian counterparts.

“It’s shocking and unforgivable that the Iranian authorities can callously force prisoners of conscience into starving themselves in protest at their plight,” said Ellie Kennedy, an Individuals at Risk Campaigner at Amnesty International UK.

“It should obviously never have come to this,” she added.