Mining billionaire Andrew Forrest meets Pope Francis to discuss a bid to end slavery. "This isn't something I've tripped over; it's something I got dragged into," he said. Five years ago his daughter Grace, aged 15, volunteered at an orphanage in Nepal – but then found out that all the children she had been looking after had been ''groomed'' for sex work. "It was horrible for [Grace]," Mr Forrest said. He set up a team to find out what happened: the children had been trafficked through India into the Middle East. Mr Forrest travelled to Asia and met some of the victims. "There was one kid there [from Kathmandu] who could only say she had been sent to a land where the men wore long dresses," he said.

A captive sex worker in the room where she lives and works on the Indonesian island of Batam. Credit:Peter Morris Her ordeal began when she was only nine, and three years later she was put on a plane back to India. "She hadn't spoken for several months. She gave her story and from that point on she would only rock on her bed and whimper,'' Mr Forrest said. "When I walked through the little columns of beds to this kid, she was wailing at the back and just rocking from side to side. The screams she let out when she saw an approaching male, and the horror and revulsion and the terror on her little face will be stuck with (my wife) Nicola and I forever."

Mr Forrest started researching the slave industry. He commanded a review of all his supply chains at Fortescue and required affidavits of all suppliers that they had reviewed their own supply chains, and to his disgust found that "three or four had trouble signing that affidavit". He felt "really awful". It led him to resign as chief executive of Fortescue, taking a position as non-executive director, and now he spends most of his time in philanthropic causes. "It all spins off that experience really - to live a useful life you've actually got to help others." In 2010 he started building Walk Free Foundation, which launched in 2012. Last year it published a Global Slavery Index, identifying the countries where the problem is most acute. He is also pushing corporations to clean their supply chains.

"You're up against defensive chief executive and chairmanship behaviour," he said. At a lunch of executives of major corporations chaired by Bill Clinton last year, Mr Forrest forced most of them to admit they had slavery in their supply chains. "Then we had a really constructive conversation," Mr Forrest said, after they realised their culpability and the risk to their public image. But there was a gap that frustrated him. "Wherever we've gone around the world we've found quite significant gaps: the holy texts, no matter which one you turn to, has ambiguity in it around slavery," he said.

"That, we knew, was being used as justification by slavers all over the world." Last year he found an opportunity to close that gap. A meeting of mining executives and charities last year at the Vatican (Forrest is Christian, though not Catholic) gave him a chance to engage with the Church on the topic. Pope Francis has identified slavery as one of the evils he is most keen to combat: in his first Easter message last year he said human trafficking was a threat to peace and he instructed the Vatican's Pontifical Academy of Sciences to focus on the issue. "He is of the view that it is one of the worst scourges of humanity," Mr Forrest said. Mr Forrest proposed an alliance between multiple faiths and his Walk Free Foundation. He got the Archbishop of Canterbury, Justin Welby, on board, and then flew to Cairo and recruited the grand imam of al-Azhar, Ahmed al-Tayyeb, one of the highest scholarly authorities in Sunni Islam.

In a major coup for the nascent Global Freedom Network, Sheikh Tayyeb agreed to join the group and issue a fatwa, or religious ruling, officially published on Monday. It declares that "the worst type of human trafficking [is] the kidnapping of women and girls", prohibits "modern-day slavery in all of its types", and commits al-Azhar to an official study, which will "clearly explain the position of Islam on slavery". "This was just unbelievable," Mr Forrest said. "There has never been an operating agreement between [Catholic and Islamic institutions] ever in history." This joint venture between the faiths will be a "major operating company", Mr Forrest pledges. The GFN's aims include: ■ All global faiths to remove any slavery-related organisations from their supply chains and investments.

■ 162 governments and 30 heads of state to endorse the GFN by the end of 2014. ■ Political leaders to slavery-proof government supply chains. ■ The G20 to adopt a new anti-slavery and human trafficking initiative. ■ 50 multinational businesses to commit to ''slavery-proofing'' their supply chains. It defines slavery broadly, including sex trafficking, forced labour and forced marriage.