A film made over 10 years with the stories of girls and women affected by female genital mutilation is tackling a taboo subject

A young girl is given a plastic bag of sweets and a bottle of lemonade after being genitally mutilated … the story of the 10-year fight against female genital mutilation by two film-makers has been made into a hour long documentary by the Guardian and BBC Arabic and will go out across the Arab world from Friday, reaching a combined global audience of 30 million viewers. This is the Guardian's shorter web version of that film

It started out as a film about a practice that has afflicted tens of millions of women worldwide. It culminated in a change in the law.

Ten years after they embarked on a documentary to investigate the extent of female genital mutilation in Kurdistan, two film-makers have found their work changing more than just opinions in a fiercely conservative part of the world. Partly as a result of the film, the numbers of girls being genitally mutilated in the villages and towns of Iraqi Kurdistan has fallen by more than half in the last five years.

Shara Amin and Nabaz Ahmed spent 10 years on the roads of Kurdistan speaking to women and men about the impact of female genital mutilation (FGM) on their lives, their children and their marriages. "It took a lot of time to convince them to speak to us. This was a very taboo subject. Speaking about it on camera was a very brave thing to do.

"It took us weeks, sometimes months to get them to talk and in the end it was the women that spoke out – despite the men," said Ahmed.

The result was a 50-minute film, A Handful of Ash. When it was shown in the Kurdish parliament, it had a profound effect on the lawmakers.

The film-makers' work began in 2003, shortly after the fall of Saddam Hussein. The stories they were told had a numbing consistency. In one scene in the documentary a young mother with her children sitting beside her tells Shara that in their village: "They would just grab the little girls, take them and cut them, and the girls came back home. I can still remember I was sick, infected for three months. I could barely walk after I was cut."

A mullah tells the film-makers that "Khatana [the Kurdish term for FGM] is a duty; it is spiritually pure." That is the position of the Shafi'i school of Sunni Islam that is practised by Iraqi Kurds. It is the same branch of Islamic law that predominates in Egypt, where studies show that up to 80% of women have been mutilated. But FGM is not just confined to some Muslim countries in the Middle East – it is also widespread in parts of Africa and

Indonesian. It pre-dates Islam or Christianity and is on record since

the time of the Pharaoh.

"It is about controlling women's sexuality and keeping them under control," said Nadya Khalife, from Human Rights Watch.

There are an estimated 140 million girls and women worldwide living with the effects of FGM, the World Health Organisation estimates.

"There were a lot of kids in the room," one 18-year-old woman told the film-makers. "My mother and sister took hold of me. They were taking off my trousers and separating my legs. I screamed 'What are you doing to me?' My mother said 'nothing dear, just a little pain'. They put me on the ground and the pain started between my thighs. Everything turned dark. When they wanted to raise me to my feet, I couldn't stand. My thighs were covered in blood."

Another woman took them to her sister's grave. "One of my sisters got an infection and died. She was cut with a dirty blade. She had an infection for two days so we took her to a doctor. He couldn't treat her and she died. She is buried here."

"It's not something that families discuss. It's just something that is done, and is forgotten about," said Khalife. "Countless generations of girls were sentenced to lives lacking in sexual pleasure or fulfilment and cheerless marriages."

One Kurdish couple encountered by Amin and Ahmed illustrated this underlying sadness with extraordinary, raw honesty.

Hawa, a seamstress, and Erat, a farmer, have been married for 10 years and have three children. Hawa said she and her three sisters were mutilated at her grandmother's insistence.

"My two sisters and I, three of us, we all had khatana [cutting]. Believe me, my mother did not care about the practice, never insisted. But my grandma insisted. She would always say food and water served by their hands would be haram [impure] if the girls were not cut."

Asked about the extent of her mutilation, she said: "My husband always says 'nothing is left of you'."

Hawa's husband said FGM had destroyed his marriage. "I was not aware of this when I married her. If I had known, I swear to God even if they paid me $10,000 I would not have married her. Because it is a problem for me.

"This circumcision is similar to neutering animals," he said. "It's a major problem. There is no sensation. It feels like lying next to a cold fish."

Piroza, now 15, recalled what was done to her when she was five. "They said: 'Come here, we brought beads for you.' They took me into a room. There was an old woman. There was a razor and ash, and they cut me."

The film-makers found that when they first started the project they were visiting villages where every one of the girls had been mutilated. The Iraqi Kurdish government denied that it was widespread but the film-makers' testimonies found otherwise. In many cases, all the women in a village had been mutilated – usually between the ages of five and nine. Most alarmingly, the fall of Saddam Hussein had led to a resurgence of the practice – FGM was seen as a mark of national cultural independence for the Kurds.

The film-makers, both in their 30s and ardent campaigners against the practice, joined forces with Wadi, a small German-Iraqi non-governmental organisation dedicated to eliminating FGM in Iraqi Kurdistan, and took their campaign film to parliament. They were invited in but only female politicians turned up to the viewing. Nevertheless, the showing sparked a campaign by the Kurdish parliamentary women's committee to outlaw FGM.

It took three years and it was not until Human Rights Watch published a devastating report into the scale of FGM in Kurdistan, and pressure was applied on the Kurdish government from Brussels, that the law was implemented in 2011.

It then sparked a debate with the Muslim clergy. A key turning point was when a leading Kurdish cleric, Mullah Omar, told a conference organised by Human Rights Watch: "Female circumcision is an injustice. It is a crime against women." A fatwa, or edict, was declared against it and word began to filter down to the villages.

One midwife who practised FGM for 20 years, Pura Sewa, said: "We have been advised not to practice mutilation and we have obeyed that. They said not to mutilate or you will be taken to jail. They took away my licence and I stopped. But if they hadn't taken it away, I would still be performing khatana for Islam."

Female genital mutilation – the facts:

• Female genital mutilation (FGM) is carried out in 29 countries, according to the World Health Organisation. Although prevalent in many Muslim countries in the Middle East, it is also widespread in Africa and Indonesia.

• Some 140 million girls and women globally currently live with the consequences of FGM, according to the WHO.

• FGM is mostly carried out on young girls sometime between infancy and the age of 15.

• More than 18% of all FGM is performed by medical professionals – and this trend is increasing.

• FGM varies from the the cutting the clitoris in some countries, such as Kurdistan, to removing all the external genitalia.

• More than 66,000 women and girls living in Britain have undergone FGM, according to the NHS, but there has not been a successful UK prosecution since it was criminalised 28 years ago.

• Human Rights Watch calls FGM a practice to control women's sexuality.

• There are several degrees of mutilation. In the most extreme form, the sexual organs are removed and the vagina is sewn up and narrowed. It is thought to reduce a woman's libido and thus to help her resist "illicit" sexual acts.

• Long-term consequences can include infertility and an increased risk of childbirth complications and deaths of newborns. Complications can also result in the need for later surgery. • In December 2012 the UN general assembly approved a resolution calling for the elimination of FGM.

• In most countries the prevalence of FGM has fallen, and an increasing number of women and men where it is practised support ending the practice.