A British woman has been locked up in Iran for five months after posting derogatory comments about the country's government on Facebook and fears she will be executed, her husband has said.

Concerns are growing for the welfare of Roya Saberi Negad Nobakht, 47, from Stockport, who has been charged with "insulting Islamic sanctities", a crime which can be punishable by death. The Foreign and Commonwealth Office (FCO) said it was "urgently" looking into her case. However, Britain currently has no embassy in Iran, making any negotiations more difficult.

Mrs Nobakht was in Iran visiting family in October last year when she was arrested by police as she arrived by plane in the south western city of Shiraz, according to an account given by her husband, Daryoush Taghipoor, to a family friend in Britain.

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She was then taken back to Tehran and charged with "gathering and participation with intent to commit crime against national security" and "insulting Islamic sanctities", according to a copy of her charge sheet seen by The Independent.

Mr Taghipoor, who is currently in Iran, claimed that his wife's arrest was over comments she had made on a Facebook group about the government being "too Islamic", and that she had only been charged after a confession was extracted from her "under duress". Facebook declined to comment.

She is believed to have been detained ahead of a trial at Evin prison in Tehran, which is notorious both for its harsh conditions and for its housing of political prisoners.

The couple have lived in the Stockport suburb of Heald Green for the past six years and have both been granted dual British-Iranian nationality. Mrs Nobakht had previously been a student at Stockport College, but a spokeswoman said today she had left four years ago.

Mr Taghipoor told the Manchester Evening News: "It's a very bad situation. We don't know what's going on. Roya is not well at all. She has lost three stone and is frightened. She is scared that the government will kill her."

Mrs Nobakht's situation came to light after her husband approached his friend and former employer Nasser Homayoun-Fekri, who also lives in Stockport, who wrote a letter to his local MP Andrew Stunell.

"I must request, on humanitarian grounds as well as for the sake of justice, that you exert all possible pressure on the British Government to do all that is feasible for the release of this innocent British citizen," he wrote. "Especially considering that the Islamic Republic, as acknowledged by the United Nations… is one of the most notorious human rights abusers at all the stages of arrest, custody and trial."

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Mr Homayoun-Fekri told The Independent he had spoken to Mr Taghipoor on Tuesday but that his telephone line had since been cut off. "He used to work with me. He is not into politics or opposition groups, and neither is his wife," he added. "I was very surprised when he told me she had been arrested."

Mr Stunell said he had "every reason to believe" that Mr Taghipoor's account was true and that he had asked the FCO to seek consular access to Mrs Nobakht. However, he said this was proving difficult as the British Embassy in Tehran was closed and a third party had to act as a go-between.

"I don't think anybody knows exactly and precisely what she's been arrested for," he added. "The randomness of these things is one of the areas of concern. But what she's been charged with appears to be an offence against attacking the holiness of Islam, which gives substance to [internet postings] being the reason."

Human rights groups have expressed concern about the reasons for Mrs Nobakht's arrest. Amnesty International's Iran researcher Bahareh Davis said: "If Roya Saberi Nejad Nobakht has been held solely for peacefully exercising her right to freedom of expression she must be released immediately and unconditionally.

"Regrettably, vaguely-worded and broadly-defined crimes such as 'gathering and colluding against national security' and 'insulting the Islamic sanctities', for which she appears to have been held, are often used by the Iranian authorities to curb those who peacefully express their opinions, including criticising the government."

Faraz Sanei, Iran researcher at Human Rights Watch, said his organisation had documented "many cases of political activists detained on trumped up charges" and that Iran had previously "used public information on Facebook and social media sites and other electronic outlets to charge individuals with insulting the government".

The FCO said in a statement: "We are aware of reports [about this case] and are looking into them urgently."

Despite the lack of a British embassy in Tehran, relations between the two countries appear to have thawed recently. In January, former Home Secretary Jack Straw headed the first UK delegation to travel to Iran in five years, accompanied by former Chancellor Lord Lamont, Conservative MP Ben Wallace and Labour MP Jeremy Corbyn as guests of the Iranian Parliament.

Mr Straw said on his return he felt there was a "lighter atmosphere" in the country, adding he believed President Hassan Rouhani was "committed to change" and would like to bring it "in from the cold".

Evin prison: Far less pleasant than the name suggests

Evin jail in the northern suburbs of the Iranian capital, Tehran, was once given the moniker "Evin University" due to the number of political prisoners housed there. But the reality of the prison, which was constructed in 1972, is far less pleasant than the name suggests.

Standing at the foot of the Alborz Mountains, it is home to an estimated 15,000 inmates, including murders, thieves and rapists as well as intellectuals and dissidents. Former inmates have complained of human rights abuses such as beatings, torture and mock executions.

Hamid Ghassemi-Shall, a Canadian-Iranian businessman, was released from Evin in September last year after being convicted of "espionage" and sentenced to death. He told Amnesty International how he was blindfolded, dragged up a flight of stairs and made to sit against a wall in silence, convinced he was about to be killed.

One of the jail's most notorious cases of abuse involved Zahra Kazemi, a Canadian-Iranian photographer who was arrested in 2003 after taking pictures of her relatives outside Evin. She was later taken to hospital with severe injuries and later died. The Iranian government claimed she had suffered from a stroke while being interrogated.