Ten years ago, a young Mandarin specialist at GCHQ, the government’s surveillance centre in Cheltenham, did something extraordinary. Katharine Gun, a shy and studious 28-year-old who spent her days listening in to obscure Chinese intercepts, decided to tell the world about a secret plan by the US government to spy on the United Nations.

She had received an email in her inbox asking her and her colleagues to help in a vast intelligence “surge” designed to secure a UN resolution to send troops into Iraq. She was horrified and leaked the email to the Observer. As a result of the story the paper published 10 years ago this weekend, she was arrested, lost her job and faced trial under the Official Secrets Act

The memo from Frank Koza, chief of staff at the “regional targets” section of the National Security Agency, GCHQ’s sister organisation in the US, remains shocking in its implications for British sovereignty. Koza was in effect issuing a direct order to the employees of a UK security agency to gather “the whole gamut of information that could give US policymakers an edge in obtaining results favourable to US goals or to head off surprises”. This included a particular focus on the “swing nations” on the security council, Angola, Cameroon, Chile, Bulgaria and Guinea, “as well as extra focus on Pakistan UN matters”.

The story went around the world and the leak electrified the international debate during the weeks of diplomatic deadlock. Most directly, it bolstered opposition to the US position from Chilean and Mexican diplomats weary of American “dirty tricks”. The same countries demanded immediate answers from the British government about its involvement in the spying. With the operation blown, the chances of George W Bush and Tony Blair getting the consensus for a direct UN mandate for war were now near zero.

For the Observer too, it was a story full of risks. The paper had taken the controversial decision to back intervention in Iraq. Yet here was a story that had the capacity to derail the war altogether. It remains entirely to the credit of Roger Alton, at the time the paper’s editor, that he stuck with the story, despite its potential implications.

Gun had hoped the leak would prick the conscience of the British public, large sections of which were already taking to the streets in opposition to the war. Surely, she thought, when people realised that the UK was being asked to collaborate in an operation to find out personal information that could be used to blackmail UN delegates, they would be outraged and the UK government would halt its slide into war. She failed.

A decade on, sitting in a cafe in Cheltenham, not far from GCHQ, I asked her if she still stood by what she had done. “Still no regrets,” she said. “But the more I think about what happened, the more angry and frustrated I get about the fact that nobody acted on intelligence. The more we find out that in fact the million-person march was a real cause of worry for Downing Street and for Blair personally, it makes you think we were so close and yet so far.”

Since 2003, my life and Gun’s have continued to cross from time to time. As one of the journalists who broke the story, I feel a certain responsibility for how things have turned out. Gun sacrificed so much when she decided to leak and has worked only intermittently since. Throughout her own court case, what only a few knew was that she was also fighting for the right of her husband, who is from Turkey, to remain in the UK. She now has a four-year-old daughter who she is bringing up in Turkey.

Times have often been tough, not least because of the itinerant life she has chosen for herself. “Financially it’s the toughest,” she said. “But that’s partly my own fault because I haven’t aggressively pursued a career. Because I’m not ambitious it’s not paramount for me to find myself in a high-paid job. Right now my priorities are to ensure I am there for my daughter.”

Now there is the possibility that Gun’s singular life will be made into a movie. A script has been doing the rounds in Hollywood for five years. It is written by Sara and Gregory Bernstein, a California-based husband-and-wife writing partnership that have worked with British director Jonathan Lynn. Everyone involved assumed the project had run into the dust, but then it appeared on the Black List, a Hollywood website for unmade film scripts, which has featured Slumdog Millionaire and The King’s Speech in the past. There it was spotted by Debs Paterson, director of the critically acclaimed Africa United, who met Katharine Gun last week with a view to making the film of her life.

Gun was visiting friends and family in Cheltenham when I talked to her, with the strain obvious on her face but still looking much younger than her 38 years. I don’t think I’ve ever met a more determined character and she remained utterly convinced of the justice of her cause: “There’s nothing subsequent to the invasion that makes me think it was the right decision made by Bush and Blair.” However, she is not without disappointment about how little obvious difference she made.

“I never aligned myself specifically with the anti-war movement. But I talk to people and there does seem to be a sense of failure that, despite all the campaigning and all the marching and all the protesting and everything they did, it made not a ha’porth of difference. And that’s quite a depressing place to find yourself in when you feel so strongly and passionately about something

As for her own story, she recognises that 10 years on it scarcely registers with the public. I sensed a slight flash of anger as she said: “It’s not even a footnote in the history of Iraq.” But she said she would still be prepared to give evidence to the Chilcot inquiry into the Iraq war. “There seems to be this blasé attitude – the spying goes on, everyone does it and so it’s nothing to get all hot under the collar about. But this specific instance is the ugly truth of what goes on.”

Although the story made headlines around the world at the time of the leak and later at the time of her trial, which collapsed after the prosecution withdrew its evidence, it remains largely missing from the official narratives of the build-up to the Iraq war.

Jack Straw, then the foreign secretary, has not been challenged on whether he authorised the operation to go ahead, although it is almost certain that he did.

Gun said that the UK government still had some explaining to do: “I think there need to be more questions asked about whether they responded to that request, why they felt it was within their scope of work to respond to that sort of request, and what is the manner of the relationship between UK politics and US politics.

It was almost as if that request was asking for someone within their own nation to do this work; it wasn’t asking another completely independent state for co-operation.”

The risks Gun took in revealing the UN email’s existence were huge. By printing off the memo, putting it in her handbag and taking it home, she was already committing a serious breach of the Official Secrets Act.

In leaking it to the Observer, she was also doing something unprecedented in the history of espionage. Not only was the cable the most sensitive ever to be disclosed on either side of the Atlantic, it was also unique in its timing. Most whistleblowers leak after the event to expose perceived wrongdoing. Gun disclosed details of the spying operation as it was happening to stop something she viewed as terrible happening in the future.

As the title of the film script suggests, she was “The Spy Who Tried to Stop a War”. Daniel Ellsberg, the celebrated American whistleblower who leaked the Pentagon Papers about the build-up to the war in Vietnam, described it as “the most important and courageous leak I have ever seen”.He added: “No one else – including myself – has ever done what Katharine Gun did: tell secret truths at personal risk, before an imminent war, in time, possibly, to avert it.”

As it was, a second UN resolution directly to authorise war against Iraq never materialised and airstrikes began on 19 March 2003. Katharine Gun did not stop the war, but was it all entirely in vain? It is probably still too early to tell.

Courtesy-The Guardian, UK