"I knew I had dynamite on my hands," says director Dror Moreh. He is talking about his Oscar-nominated documentary The Gatekeepers which has provoked wide international debate across the political spectrum since its release. Even Israeli embassies have had to grapple with how to respond to its frank revelations, admissions and insights. The Gatekeepers, wrote Israeli journalist Chemi Shalev, 'is like a waterboarding of the soul'.

The film comprises a series of extraordinary in-depth interviews with six former heads of the Shin Bet, Israel’s domestic intelligence and security service, also known as the Shabak. The six - who have never been interviewed about their work on camera before - speak with remarkable candour about their role as protectors of the Israeli State since 1967.

Supported by computed generated imagery (CGI) and archive footage, the film provides an overview of the country’s policies in the occupied territories, the agency’s counter-terrorism campaigns and the wider Israeli-Palestinian conflict.

There is no spy glamour about their work. They speak about torture, targeted assassinations and personal and political moral dilemmas. But each comes to the same conclusion: the current status quo regarding the occupation cannot continue.

"You can’t make peace using military means. Peace must be built on a system of trust," says Avi Dichter, who headed the Shin Bet between 2000 and 2005.

‘As a filmmaker in Israel I want to create films that will change reality.’

"For Israel, it’s too much of a luxury not to speak with our enemies," asserts Avraham Shalom (1980-86). Indeed, when pushed by an off-screen Moreh, he emphasises that he means, "everyone, so it includes even Ahmadinejad, whoever… I’m always for it."

These security giants have served their country since their army service until their retirement from the Shin Bet, but "they feel that the security of Israel is deteriorating all the time and the cause that they spent their life trying to achieve is getting further and further away," explains Moreh, speaking in London ahead of the film's UK release on Friday (April 12).

He was inspired to make The Gatekeepers while he was filming Sharon (2008), a documentary about the politician Ariel Sharon. During the film Moreh had interviewed Dubi Weisglass, Sharon’s Chief of Staff and one of his closest advisers. Weisglass told him that Sharon had been deeply influenced by an interview with four heads of the Shin Bet that had appeared in Yedioth Ahronoth, Israel’s most popular newspaper in 2003.

They stated that if Sharon continued to run Israel as he was, it would lead the country into disaster. Weisglass said that these remarks had greatly influenced Sharon, precisely because they came from the heart of the defence establishment, from people who were dealing with the Palestinians all the time and knew them well.

Moreh says: "As a filmmaker in Israel I want to create films that will change reality." In Sharon he wanted to explore how and why the right-wing politician shifted his thinking. He recalls hoping that his Gatekeepers film idea might have a similar effect on others, "in a way like the interviews did on Sharon. And I was right," he laughs.

He initially approached former politician and Shin Bet head Ami Ayalon, who then helped him to recruit the other directors.

But will anyone listen to these six men? Morah recounts a tale that he confesses made him very happy. After Israel’s latest election - an election result that Moreh refers to as Netanyahu’s defeat (he lost 12 seats in the Knesset) - an IDF Spokesman was questioned about the causes for this result. One of the reasons given was The Gatekeepers.

Also cited was an interview Moreh did with Yuval Diskin, one of the Shin Bet heads (2005-2011) two weeks prior to the election, which was printed in Yedioth Ahronot. "So," Moreh says, "I think it does have an impact." But, not for the far right he claims. "They are lost. Nothing will persuade them. Nor the far left."

There were numerous occasions during filming when Moreh was shocked by what he heard but more profoundly he discovered that the Israeli narrative he had grown up with - that the Israelis always wanted peace and the other side always refused - was not quite so.

"The Israelis and the Palestinians never missed an opportunity to miss an opportunity," he says, rephrasing Abba Eban’s famous quote. "This is the tragedy of the conflict because a lot of people who could be with us now are buried in Israeli and Palestinian cemeteries."

Conversation turns to discussing the chapter in the film that addresses the events surrounding Yitzhak Rabin’s death, and the visible distress that Carmi Gillon (1994-96) displays when talking about it.

Moreh’s voice drops:"It was the most devastating moment in my life." He explains that November 4, the date of Rabin’s assassination, is his birthday. "Every time I see that sequence - around 3000 times - I have tears in my eyes."

He and Gillon were recently in Los Angeles conducting a Q&A session together for the first time; someone posed a question about it and Moreh says that he almost started to cry and had to pass the microphone over to Gillon.

He says: "If there is someone who understands the consequences of that assassination, it’s Gillon. He was the one who was head of the organisation that was supposed to protect the Prime Minister, and he failed. He will always carry that."

Moreh is openly critical of Netanyahu and sees contradiction in his rhetoric. He is also unequivocal when it comes to the extreme right.

"They are the most danger to Israel. If they think that to maintain the occupation is pro-Israel, then they are gravely mistaken. If there is something damaging the state of Israel, it is the maintenance of that policy." His belief and that of the gatekeepers is that "it is enough. It is devastating us from within."

Moreh plans to have The Gatekeepers subtitled into Arabic and wants it shown in the West Bank. He hopes the film will also reach Gaza. A five-part series is being made for Israeli TV - Moreh has over 70 hours of material - as well as a book.

For Moreh, the political future is bleak. Israelis and Palestinians alone cannot resolve the conflict. Pressure, he says, must come from outside, from the EU and America. He has little faith in the current Israeli political climate.

"The task of leaders is to lead to a better solution. At least honestly strive for that."