On a grey, drizzly Saturday afternoon at Reading Festival a colourful collection of superfans are gathered. They look oddly out of place amongst the smattering of lads, Metallica fans and denim-shorted festival regulars. Some are decked out in the black and red T-shirts bearing the name of their favourite band while a few of them have gone all out with some bizarre make-up. But all of them have one thing in common: they’re here to see Babymetal.

Babymetal are three Japanese teenage girls backed by a six piece band. The girls may be giggly ‘idols’ – the quintessentially Japanese phenomenon of young, manufactured pop starlets sold as being cute – but their genre is metal. The resulting sound is a mix of poppy vocals, the occasional DragonForce bleep and double bass-pedals, played at a frenetic pace.

Whatever your tastes, what can’t be doubted is their popularity, which has long since spread from their home country. In February last year their eponymous debut album sold 37,000 copies in its first week in Japan before topping the iTunes metal charts in the US, UK and Germany. Two serious moshpits developed at their spot at Sonisphere in 2014 and in April next year they’ll headline Wembley Arena, arguably the most sought-after spot for metal bands performing in the UK.

When I sidle over to the group with my iPhone, they are more than happy to talk. Whilst Reading can sometime feel populated by the image-concious, these guys couldn’t care less. When I ask someone to explain to me what it’s all about, I’ve no shortage of takers.

“They just have the most energetic live shows around at the moment,” says David, a 23-year-old Reading local. “They’re just so different, really fun and nothing like you’ve ever seen before. There are people [from Japan] who have just come here and then they’re going up to Leeds as well to see them there. Some people have already left here to go up there.”

Scott, a softly-spoken 22-year-old who has travelled from Stirling, agrees. “It’s definitely a phenomenon. When they released their album last year they just started to become really popular. I read the other day that it took Avenged Sevenfold fourteen years to headline Wembley Arena, and Babymetal have done it basically in a year and a half.”

Japanese metal culture, of course, is nothing new. But Babymetal, through their energetic live shows and music chops displayed by the band, seem to have attracted respect from European metal fans and Japanese pop idol-culture devotees in equal measure. This achievement is illustrated perfectly by the group of fans at Reading, who are of all ages and several continents.

Toby is a 29-year-old German who is literally following the band all over the world. “This year I’ve followed their whole European tour, so Germany, Switzerland, Italy and Austria. Then two weeks later I went to Tokyo, and now I’m here to see them at Reading.

“They are quite simply the best band in the world right now,” he says with authority, as his friends nod in agreement. “It’s the group, but it’s also the fans. We’re like a big family. We always arrive six, seven hours before the show starts to have a party.”

There are also several Japanese superfans with them who have travelled halfway across the world to see their favourite band. One of them (who later disappears before I have a chance to catch his name) explains that Babymetal have broken out of simply being an idol act in Japan.

“Pop idol culture is huge in Japan,” he explains in slightly broken but confident English, “so they have a particular type of fanclub. First Babymetal were just [idols] but now they are becoming more and more popular as a band. The new shows are the best they have ever done and they’re getting better all the time.”

His friend chips in at this point, though needs my first Japanese interviewee as a translator. “When I was a teenager I was a metalhead,” he says. “I see metal essence in Babymetal’s music, which is why I love them”.

I take some pictures of them and drift away. A few hours later, Bring Me The Horizon take to the main stage. Frontman Oliver Sykes emerges from a fog of smoke… wearing a Babymetal T-shirt. It looks like they could be around a while yet.