Amal Farah, a 32-year-old banking executive, is laughing about a contestant singing off-key in the last series of The X Factor. For a woman who was not allowed to listen to music when she was growing up, this is a delight. After years of turmoil, she is in control of her own life.

On the face of it, she is a product of modern Britain. Born in Somalia to Muslim parents, she grew up in Yemen and came to the UK in her late teens. After questioning her faith, she became an atheist and married a Jewish lawyer. But this has come at a cost. When she turned her back on her religion, she was disowned by her family and received death threats. She has not seen her mother or her siblings for eight years. None of them have met her husband or daughter.

“It was the hardest thing I’ve ever done – telling my observant family that I was having doubts. My mum was shocked; she began to cry. It was very painful for her. When she realised I actually meant it, she cut communication with me,” said Ms Farah. “She was suspicious of me being in contact with my brothers and sisters. She didn’t want me to poison their heads in any way. I felt like a leper and I lived in fear. As long as they knew where I was, I wasn’t safe.”

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This is the first time Ms Farah has spoken publicly about her experience of leaving her faith, after realising that she did not want to keep a low profile for ever. She is an extreme case – her mother, now back in Somalia, has become increasingly radical in her religious views. But Ms Farah is not alone in wanting to speak out.

It can be difficult to leave any religion, and those that do can face stigma and even threats of violence. But there is a growing movement, led by former Muslims, to recognise their existence. Last week, an Afghan man is believed to have become the first atheist to have received asylum in Britain on religious grounds. He was brought up as a Muslim but became an atheist, according to his lawyers, who said he would face persecution and possibly death if he returned to Afghanistan.

In more than a dozen countries people who espouse atheism or reject the official state religion of Islam can be executed under the law, according to a recent report by the International Humanist and Ethical Union. But there is an ongoing debate about the “Islamic” way to deal with apostates. Broadcaster Mohammed Ansar says the idea that apostates should be put to death is “not applicable” in Islam today because the act was traditionally conflated with state treason.

Some scholars point out that it is against the teachings of Islam to force anyone to stay within the faith. “The position of many a scholar I have discussed the issue with is if people want to leave, they can leave,” said Shaykh Ibrahim Mogra, the assistant secretary general of the Muslim Council of Britain. “I don’t believe they should be discriminated against or harmed in any way whatsoever. There is no compulsion in religion.”

Baroness Warsi, the Minister of State for Faith and Communities, agreed. “One of the things I’ve done is put freedom of religion and belief as top priority at the Foreign Office,” she said. “I’ve been vocal that it’s about the freedom to manifest your faith, practise your faith and change your faith. We couldn’t be any clearer. Mutual respect and tolerance are what is required for people to live alongside each other.”

Yet, even in Britain, where the freedom to change faiths is recognised, there is a growing number of people who choose to define themselves by the religion they left behind. The Ex-Muslim Forum, a group of former Muslims, was set up seven years ago. Then, about 15 people were involved; now they have more than 3,000 members around the world. Membership has reportedly doubled in the past two years. Another affiliated group, the Ex-Muslims of North America, was launched last year.

Their increasing visibility is controversial. There are those who question why anyone needs to define themselves as an “ex-Muslim”; others accuse the group of having an anti-Muslim agenda (a claim that the group denies).

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Maryam Namazie, a spokeswoman for the forum – which is affiliated with the Council of Ex-Muslims of Britain (CEMB) – said: “The idea behind coming out in public is to show we exist and that we’re not going anywhere. A lot of people feel crazy [when they leave their faith]; they think they’re not normal. The forum is a place to meet like-minded people; to feel safe and secure.”

Sulaiman (who does not want to reveal his surname), a Kenyan-born 32-year-old software engineer living in East Northamptonshire, lost his faith six years ago. His family disowned him. “I knew they would have to shun me,” he said. “They are a religious family from a [close] community in Leicester. If anyone [finds out] their son is not a Muslim, it looks bad for them.” He added that people “find it strange” that he meets up with ex-Muslims, but he said it is important to know “there is a community out there who care about you and understand your issues”.

Another former Muslim in her late twenties, who does not want to be named, said the “ex-Muslim” identity was particularly important to her. “Within Islam, leaving [the religion] is inconceivable. [The term] atheist doesn’t capture my struggle,” she said, adding that her family does not know the truth about how she feels.

Pakistani-born Sayed (not his real name), 51, who lives in Leeds, lost his faith decades ago. He left home at 23 and moved between bedsits to avoid family members who were looking for him. He told his family about his atheism only two years ago. “I was brought up a strict Muslim, but one day, I realised there was no God,” he said. He told his mother and sister by letter that he was an atheist but they found it difficult to comprehend.

“Whenever I tell my sister or my mum that I am depressed, stressed or paranoid, they say it’s because I don’t pray or read the Koran enough,” he said, adding that he will not go to his mother’s funeral when she dies. “I won’t be able to cope with the stress or the religious prayers. There’s quite a lot of stigma around.”

Iranian-born Maryam Namazie, 47, said that it does not have to be this way. Her religious parents supported her decision to leave their faith in her late teens. “After I left, they still used to whisper verses in my ear for safety, but then I asked them not to. There was no pressure involved and they never threatened me,” she said. “If we want to belong to a political party, or religious group, we should be able to make such choices.”

Raheem, 26, from London, is another who doesn't wish to use his real name. He has not yet told his parents that he is an atheist. Born into a traditional Pakistani family, he said he knew he didn’t believe in God from the age of 15.

“Most people transition out of faith, but I would say I crashed out. It was sudden and it left a big black hole. I found it hard to reconcile hell with the idea that God was beneficent and merciful.

“I’m sort of worried what will happen when [my parents] find out. For a lot of older Muslims, to be a Muslim is an identity, whereas, for me, it’s a theological, philosophical position. They might feel they have failed as parents; some malicious people might call them up, gloating about it. Some would see it as an act of betrayal. My hope is that they will eventually forgive me for it.”