Image copyright volteface

"I've had a samurai sword to my throat, a knife in my groin, stripped naked at gun point." For 14 years, Neil Woods infiltrated UK drug gangs for the police, at great personal risk.

Posing as an addict to infiltrate some of the UK's biggest drug gangs is not for the faint-hearted.

It takes resolve, courage, and an ability to think on your feet in the most high-pressure of environments.

But for Neil Woods, it was not a life calling that led him to devote 14 years to this cause, but a failure to perform in his normal role.

"I wasn't a very successful uniform cop," he tells the BBC's Victoria Derbyshire programme.

"I struggled, so I got an attachment with the drugs squad. They suggested trying one of the undercover jobs of buying crack cocaine."

It was 1993, and this form of undercover work was rare in the UK.

For Mr Woods, however, it was a niche that played to his strengths.

"I really enjoyed the work, I found I was good at it," he says.

"I was developing the tactics for it - such as building a cover story, but not acting. Learning to play a different version of yourself.

"It all relies on empathy - 'weaponising' empathy to get close to them."

Image copyright Neil Woods Image caption A shot of Neil Woods undercover, in around 1995

Mr Woods admits he had a "completely prejudiced view" of drug users when he first took the job, but as he began to meet addicts, he saw a different side to those who had been caught up in that world.

"Beforehand, I saw them as people who had made the wrong decision, who didn't have willpower. I thought that it was their fault," he says.

"But then you start to realise some of their life stories - that they had been self-medicating for child abuse, for example.

"Two-thirds of heroin users have a history of abuse."

At the time, however, this did not alter his approach to work.

"I still carried on manipulating them," he says.

"They were collateral damage.

"They would get wrapped into the investigation and end up in jail.

"I justified to myself that the end would justify the means."

The reason for this becomes clear when you consider the type of gang members he was looking to put behind bars.

In 2004, he helped bring six members of the notorious Burger Bar Boys to justice.

They operated in the Birmingham area, and were "horrendous criminals", according to Mr Woods.

"They were raping people as punishment for drug debt," he says.

Image copyright Neil Woods Image caption Mr Woods spent 14 years undercover

Mr Woods worked across inner cities around the UK, and often found himself in dangerous situations.

"I've had a samurai sword to my throat, a knife in my groin, stripped naked at gun point," he says.

"Once, my hidden camera was found by a particularly vicious gangster.

"He brought two mates to a meet-up who didn't know me.

"They searched me and found the camera.

"I had to react quickly, I just launched into a torrent of abuse.

"It created confusion so I could escape.

"Then they came after me in the car and they tried to run me over.

"I later learned that they had a gun in the car."

Mr Woods says his work has led to more than a combined 1,000 years of jail time for the criminals he helped to lock away.

When he became a father, however, he says he found it difficult to juggle his job with his family life.

"I would do this work in the week, then at the weekends be a dad. I would go swimming with my kids," Mr Woods says.

Eventually, after 14 years, he decided to leave the profession.

He came to see his work as futile, given the greater picture.

"I interrupted the drug supply for no more than two hours in any city. So what's the point?" he says.

"Some of those arrested were organised criminals - but many were just victims of the 'war on drugs', the vulnerable problematic users."

For several months, he suffered from post-traumatic stress disorder (PTSD) as a result of what he had seen undercover.

Image copyright Science Photo Library Image caption Mr Woods is in favour of Durham Police's plans to provide heroin to addicts in medically supervised conditions

Despite leaving the police, however, he still feels "duty bound" to continue to stop the spread of drugs on UK streets.

Mr Woods is now the chairman of Leap UK, which campaigns for drugs policy reform.

It supports Durham Police's plan to give heroin addicts the class-A drug in supervised "shooting galleries" in a bid to tackle drug-related crime.

Opponents say trials suggest such initiatives do not have significant, long-term benefits, but Mr Woods argues the move will enable police to "get a grip on heroin, get it away from criminals".

"The drug supply is currently in the hands of organised criminals," he says, "it's so dangerous."

It is very different from his former job, but - as he explains - his time undercover has made a lasting impression.

"I have this unique experience," he says. "Now, I just use it in a different way."

Watch the Victoria Derbyshire programme on weekdays between 09:00 and 11:00 on BBC Two and the BBC News channel.