As someone so utilised, he was used against some major criminals, though most of his assignments seem to have been against wider criminal fraternities. So, for example, he worked undercover in the Moss Side in Manchester, building a picture of the criminal landscape there, and again later in Cambridge. What he never seems to have done is been deployed against a major criminal or crime family, such as the Adams crime family in London, or the Noonan crime family in Manchester. So, he never seems to have been deployed to infiltrate such an OCG and “bring them down”. This isn’t a criticism of Shay, or even the police. As a layperson, I don’t know. Perhaps that’s not how it’s done, or such an operation would be too risky.

He describes it as a game of "high-stakes chess", which required hypervigilance at all times. "My expertise was starting from scratch," he says of his life in the shadows. Shay Doyle grew up on a tough Manchester council estate where drugs and gangs were rife. A life of crime would have been an easy path to take. So it went against everything that was expected of him when he joined the police. All the years he had spent learning the ways of the street: how to talk, what to say, how to conduct himself in the company of criminals proved to be gold dust. "Most cops speak in cop language, even when they are off duty - any decent criminal can spot them a mile off," he says. That said, some of his work (albeit not his undercover work so much, but his intermittent work in the organised crime unit) led him to take on Dale Cregan and the OCG he was part of, after Cregan murdered two police officers, and then later, to pursue Paul Massey’s killers.Soon he was headhunted by GMP's specialist undercover unit and tasked with infiltrating the city's underworld. All of his old life was removed and he was given a new identity - that of Belfast-raised 'grafter' and armed robber Mikey O'Brien. My calm facade belied the rage erupting inside. For 42 days he'd been at large, 42 days that ended in an act of unimaginable horror. I can still see his face. It still haunts me." The soldier-turned-undercover policeman risked his life to infiltrate south Manchester's gangland. He was also on the frontline of some of the most high profile police operations in Greater Manchester history, including the Stepping Hill poisonings and the hunt for double cop killer Dale Cregan. Nor does he view his own disillusionment as an isolated case. "Many officers in undercover policing walk away with a very bad taste in their mouths.

In the astonishing book he has written about his covert career, Doyle explains that while surveillance is the art of seeing and not being seen, undercover work - especially cold, long-term infiltration - is the art of "being seen and using it to your advantage". Raised on the streets of a 'tough Manchester council estate' Shay Doyle could easily have turned to a life of crime. Instead, he ended up playing the part of a gangster and helped bring down some of Manchester's most feared underworld figures. It wouldn't be long before Shay's prodigious talent caught the attention of the top. Then came the call that changed his an offer to join the secret Level 1 undercover unit known as Omega. And it was easy to see why they wanted him; he wouldn't have to stray too far from what he already knew. He had all the attributes of a professional criminal - the athletic physique of a cage fighter, the talk, the walk. Streetwise and fearless, he'd be a match for the most hardened villain. He was given a new identity, his DNA and fingerprints were removed from the national database, and so began the life of Mikey O'Brien. I'm proud of the work I did that kept people safe, that stopped them from being shot dead, that removed guns from the streets. That's what I signed up to do. I learned how to make cutting ­charges to blow a hole in a wall and how to breach the skin of an armoured cash van with a shape charge.Shay's police career spanned 17 years, and he was one of the UK's chosen few Level 1 undercover operatives. Certain things came easy to me, and I had an innate drive to succeed. Whatever field I went into, I would put myself up against more educated people, and it gave me fuel”.