Katharine Gun lost her GCHQ job after telling the Observer how the US tried to spy on UN delegates. Now Natalie Dormer will star in a film about her exposé

Before Julian Assange, the WikiLeaks founder, and Edward Snowden, the intelligence whistleblower, there was Katharine Gun.

Not so many people remember the unassuming British woman who once risked it all to take a stand against the US war machine, but that is about to change. The former GCHQ employee will be played by Natalie Dormer, star of The Tudors and Game of Thrones, in a film that continues the big screen’s love affair with spies and journalists.

Gun was a young Mandarin specialist at the British government’s eavesdropping agency in Cheltenham. In early 2003 she received an email asking her and her colleagues to help the US government spy on UN security council delegations in New York. It was a critical moment, as Washington was seeking UN backing for its invasion of Iraq.

Gun decided the world had to know, whatever the cost to her life and career. She leaked the memo to the Observer and was arrested, lost her job and faced trial under the Official Secrets Act.

Thirteen years later, as bloodshed continues in Iraq, the almost forgotten story is to be brought to a new audience in Official Secrets, a movie co-starring Paul Bettany and Martin Freeman as Observer journalists who reported the dirty tricks scandal, along with Anthony Hopkins as a retired general and Harrison Ford as a veteran CIA agent. It will chart Gun’s unlikely bid – courageous self-sacrifice to supporters, treachery in the view of critics – to block George W Bush and Tony Blair’s march to war.

“It’s really surreal,” Gun, 41, who was born in Taiwan and studied Mandarin and Japanese at Durham University, said last week. “It already feels like such a long time ago, like almost a different person, but to have it brought to the big screen – it will still be really weird.”

For director Justin Chadwick, whose credits include The Other Boleyn Girl and Mandela: Long Walk to Freedom, there will be the challenge of turning spycraft and journalism into drama. Film-makers keep trying, but some are more successful than others. The Watergate-inspired All the President’s Men is still widely regarded as the gold standard depiction of investigative reporting, while Spotlight, about the Boston Globe’s exposé of priests involved in sexual abuse, is nominated at this year’s Oscars.

But The Fifth Estate, which starred Benedict Cumberbatch as Assange, was a flop in 2013, costing $28m to make and taking only around $6m worldwide. Oliver Stone’s biopic of Snowden, played by Joseph Gordon-Levitt, was meant to have been released by now but has been pushed back.

Unlike many whistleblowers who leak thousands of documents after the event, Gun was intervening in an active operation and trying to stop a war with just one email. The US National Security Agency memo told employees of GCHQ to gather “the whole gamut of information that could give American policymakers an edge in obtaining results favourable to US goals or to head off surprises”. This included a focus on the “swing nations” on the security council: Angola, Cameroon, Chile, Bulgaria and Guinea “as well as extra focus on Pakistan UN matters”.

Gun did not hesitate. She printed off the memo, put it in her handbag and took it home. “I saw the email and my gut reaction was pretty instantaneous, that it was highly explosive information and that it should be out in the public domain,” she recalled. “Everybody would agree that at the time – the build-up to the invasion – the public mood across the globe was very much against an invasion, and that’s how I felt as well. There were no grounds for it, and I really felt that people needed to know what was going on behind the scenes to get a really clear picture of where our governments were taking us.”

The Observer had controversially declared its support for the war, but it verified the leak and published the story on 2 March 2003, causing an international storm. The US and UK had to give up on securing a direct UN mandate for attacking Iraq and instead launched airstrikes on 19 March, with far-reaching consequences that included a chaotic insurgency and saw the rise of Islamic State.

Gun reflected: “I think the film is important because the issues still haven’t gone away and it’s a good opportunity to focus people’s minds not only on that specific period in time but also on the ramifications of that invasion and how the ripple effect has carried on to this day. The violence, the refugee crisis, Isis – they’re all intertwined in many ways.”

Intriguing questions remain over who ultimately authorised the NSA’s request and whether it was carried out by GCHQ staff. Based on Marcia and Thomas Mitchell’s bestselling book, The Spy Who Tried To Stop A War: Katherine Gun and the Secret Plot to Sanction the Iraq Invasion, the film will try to fill some of the gaps and trace the political forces that led to the conflict.

Gun said: “There was certainly something going on at very senior levels. We know that Tony Blair was principally concerned about his reputation, about the legality of the war. The whole thing stinks to high heaven. I know people have tried to make citizens’ arrests on Tony Blair and so on, but really it’s time the international criminal court has some guts and charges white war criminals. They need to face justice just like other war criminals. I would support Bush and Blair being impeached.”

The personal cost to her was immense. After the story broke, she owned up to her bosses and was arrested. The government’s decision to charge her came as a “shock”, she recalled, and she faced a possible jail term. But as she sat in the Old Bailey awaiting the first day of her trial in 2004, the case was suddenly dropped after the prosecution withdrew its evidence. Some believe the government was anxious to avoid the embarrassment of a lengthy court case as, in the absence of weapons of mass destruction, its argument for going to war was beginning to unravel.

Gun has since kept a low profile and now lives in Turkey with her husband and seven-year-old daughter. She applauded Assange and Snowden for resisting an ever-worsening climate of official secrecy and has never regretted what she did. “It most certainly did change my life completely, but of course you don’t know how things would have panned out otherwise because I didn’t travel down that road. It’s been a completely different trajectory from what I envisaged, but I’m still here and just trying to lead a relatively normal life.”

This will also be a newspaper film. Bettany, whose CV includes A Beautiful Mind, Iron Man and Wimbledon, plays Martin Bright, the Observer journalist who came into possession of the NSA memo after meeting a contact in a cafe. Bright, now 49, from north London, said: “I was, I have to admit, sceptical. It was essentially just a sheet of paper with some words typed on it, but I had a feeling that it looked like it wasn’t made up.”

So he and two colleagues, Ed Vulliamy and Peter Beaumont, worked their contacts for a long time to stand the story up. “By the time we published we were 99% sure, but we were still having some steers from official sources that this might be a sophisticated Russian forgery, and you have to remember the time this was happening was extremely fraught ... There are a lot of risks involved and if you get something like this wrong, and if it had been a forgery or attempt to discredit the Observer, it would have been terrible for all of us.”

Bright, now a political commentator and founder of a youth employment charity, admires Gun’s determination to expose wrongdoing. “I’ve always felt it was an act of immense bravery for a young woman to take the decision to leak a document of such intense sensitivity,” he said. “This was, at the time, the highest-level leak ever.”

Screenwriters Sara and Gregory Bernstein, a California-based husband and wife, said they were drawn to the story by the courage of people like Gun and Bright and his colleagues. “Of course, once we started investigating further, we knew we had not only characters, but an explosive story. The facts here demonstrate how vulnerable we are when government officials knowingly twist information to satisfy an agenda they may sincerely believe is important.”

The script had circulated in Hollywood for years, but producer Elizabeth Fowler is confident that shooting will begin in April or May, with 33-year-old Dormer in the role of Gun. It is, she believes, a tale that needs to be told. “It is ultimately a very heroic story about a young woman who finds the courage and has the moral compass to do the right thing, solely at risk to her,” she said. “Risk only: no gain. I think that’s a tremendously inspiring thing. She said, ‘I’ve only ever followed my conscience’. Every time I hear that, tears spring up.”