As a keen teenage reader in the noughties, I was a devout fan of Anthony Horowitz’s spy series Alex Rider. “What if James Bond was a teenager?” proved a compelling premise and even the truly terrible film adaptation didn’t put me off. But on a visit to my local library, I discovered a series with a completely different angle on the life of an adolescent undercover agent. That series was called Cherub, and it was first published 20 years ago today.

The young intelligence operatives of Robert Muchamore’s books were recruited from children’s homes and troubled lives by a super-secret wing of British intelligence. Cherub agents could go where grown-ups couldn’t for the simple reason that adult crooks would never suspect children of spying.

These kids didn’t have fantastical gadgets or battle world-ending stakes like Rider, but they did have real teenage problems alongside their work infiltrating drug gangs and terror cells. They swore and snogged: in the sanitised world of early noughties YA literature, before The Hunger Games took mass child murder mainstream, it felt revolutionary.

“You’d go into the children’s bookshop and all the books are very pretty, and the kids sit cross-legged on the carpet. It’s this very cosy, very nice, very desirable world,” says Muchamore. “The only problem with that is, when you’ve got an 11 to 14-year-old boy and they’re playing PlayStation games and they’re starting to think about girls, they kind of reject that. At the time, it just really hit me that there was nothing really that appealed to the way I felt when I was a 13-year-old boy.”

The series began in 2004 with The Recruit, in which 11-year-old James Adams joined Cherub after his mum’s death and infiltrated a rural commune to foil an anthrax attack by eco-terrorists. By the time the third book, Maximum Security, came around, 13-year-old James entered the violent crucible of an Arizona supermax prison to bust out the son of a global arms dealer.

Things only got rougher. In 2006’s Man vs Beast, animal liberationists force-fed sink cleaner to a TV chef and 2007’s Mad Dogs began with a clash between rival gangs that culminated in a Cherub agent being stabbed multiple times. “I must admit there was part of me that was a bit mischievous and used to try and get away with as much as I could. It was almost like a challenge,” Muchamore says.

The books weren’t without controversy. A London junior school withdrew an invitation for Muchamore to speak and promptly banned the series, citing concerns from parents. Supermarkets also received complaints, which prompted publishers to add a “not suitable for younger readers” sticker to subsequent editions. Muchamore didn’t mind, though. “For the right kind of reader, it was actually quite cool. They quite liked the fact it had a sticker on that meant it was a bit risque. So it actually worked in all ways.”

Muchamore also separated himself from other kids’ authors by being hugely accessible to his fans. He has always answered reader emails himself and, during the peak of Cherub, he was a regular presence on a vibrant online forum dedicated to the series – a fascinating relic of the era before Facebook and Twitter were dominant.

“A lot of people slag the internet off and say it’s terrible for kids and they spend all their time and it’s harmful, and it’s dangerous,” says Muchamore. “But the fact that people from disparate social groups with marginal interests can actually go online and communicate and make friends, I think that’s amazing. I honestly think I would have been a much happier teenager if I had a group like the Cherub forum, and a bunch of people like me who had the same interest.”

While Muchamore is now focused on his series of modern Robin Hood stories, Cherub remains his biggest success. There are fans in all corners, from young offenders institutions to Hollywood – Spider-Man actor Tom Holland, who has been pictured wearing the series’ distinctive logo, revealed he loved the series in a 2014 Metro interview.

While the Cherub books might have been seen as the “anti-Alex Rider” series, Muchamore says he has had dinner with Anthony Horowitz, to have “a sit down and a chat with my so-called rival”, and found him to be “a very nice and charming chap”.

There might not be any bad blood between the authors, but James Adams would definitely want to give Alex a good slap. And my money’s on James in that fight.