Green Day frontman Billie Joe Armstrong made a very surprise appearance on stage with London-based band Borderline Toxic (Picture: Marco Waters/Touch Optical and Musical Services)

Green Day’s Billie Joe Armstrong made headlines this week as he surprised fans with a performance at a London bar.

But it’s not just the fans he surprised – the band themselves had no idea he was in the crowd, and it almost resulted in a very awkward moment.

Last week, the legendary musician popped into Slim Jims in Islington when the band performing that night, London-based rock act Borderline Toxic, launched into a cover of Green Day song Basket Case.

Billie immediately clambered up on stage, to the shock and delight of fans in the audience and on stage, with frontwoman Kerry Glass counting the star as one of her childhood heroes.

However drummer Pete ‘didn’t realise at first who it was,’ Kerry told Metro.co.uk.

‘And he almost threw a drumstrick to get him offstage,’ thinking he was simply an overzealous pub-goer.

Drummer Pete didn’t initially recognise the star and almost threw a drumstick to get him off stage (Picture: Marco Waters/Touch Optical and Musical Services)

Long-time fan Kerry, Borderline Toxic’s frontwoman, recognised her childhood hero ‘straight away’ (Picture: Marco Waters/Touch Optical and Musical Services)

‘But then he saw my face and was like, “Actually, never mind,”‘ she laughed.

Borderline Toxic played a free gig at the venue last Saturday, with Kerry explaining they didn’t usually play covers but decided to do a handful that night – and ‘one of the covers happened to be Basket Case.’

‘It was the right place, right time, right energy I guess,’ she said. ‘I would have been a nervous wreck if I knew he was there.’

The Scottish musician recalled the incredible moment Billie Joe was suddenly on stage with her, laughing:’I was already a bit p****d, and I started off singing in a Scottish accent, and the band kicked in and I looked down and there was Billie Joe Armstrong just coming up on stage with us.’

Neither the band, pubgoers nor staff haad any idea Billie Joe was in the crowd until he was suddenly on stage (Picture: Marco Waters/Touch Optical and Musical Services)

Kerry felt like the ‘luckiest person in the world’ to share the stage with the legendary rocker (Picture: Marco Waters/Touch Optical and Musical Services)

‘I realised straight away who it was, he’s rock royalty, one of my favourite bands since I was 11 years old. He said to me as he was getting up, “Is it OK?” and I went – “Er, yeah?”‘

It was pure luck the band had put a Green Day cover on their set list that night, with Kerry saying she would have been ‘absolutely fuming’ if they had only realised later that the musician had been in the bar, but admits ‘I don’t think anyone would have known he was there.’

‘He was up the back having a drink with a few folks. My mates were working the bar and they didn’t even know he was there. It would have been a case of us never knowing.’

Of course, once the star got on stage his presence was very much known, and while Borderline Toxic carried on with their set, Billie stuck around for a while to take photos with fans.

Kerry felt like the ‘luckiest person in the world’ to meet her childhood hero, recalling how she had seen them play three times in 2017 and desperately hoped to be the person Billie picked out of the crowd to sing with him on stage, as he occasionally did at shows.

It was only the band’s second gig with their new lineup, which we reckon is a good omen of things to come (Picture: Marco Waters/Touch Optical and Musical Services)

‘Fast forward five or six years and he’s up on my stage,’ she said, still in awe.

Incredibly, the chance meeting took place on what was only Borderline Toxic’s second gig with their new line-up after some original members left the band, which was first formed when Kerry and Pete began writing together during lockdown.

Borderline Toxic are now in touch with booking agents in the hopes of getting headline shows, and will return to Slim Jims on June 23 – and could even have a monthly slot in the works at the venue.

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As for whether Billie will make another appearance, well, that’s still to be seen.

But as Kerry put it: ‘It was so nice that he was in our local dive bar – that’s so punk rock!’

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One band member initially thought the legendary rocker was a stage crasher.