The four members of Abba made their first public appearance in 14 years as they attended the premiere of their Abba Voyage show in London. 

Agnetha, Frida, Benny and Bjorn reunited for the opening night of the concert, which features digital versions of the band. 

Frida watched the show with a wide grin across her face, while Benny stood up and clapped along to Dancing Queen. 

They took a curtain call at the end, to deafening applause from the audience. 

"Abba has never left us, in my heart," singer Agnetha Faltskog told the BBC on the red carpet. 

"It was not such a difficult decision [to reunite] because the music is part of us." 

"I dreamed of this for years," added Anni-Frid Lyngstad.

"We love our music, we love to sing." 

The launch night was attended by a host of musical stars including Kylie Minogue, Sophie Ellis-Bextor, Jarvis Cocker and Keira Knightley. 

Swedish King Carl XVI Gustaf and his wife Silvia also joined the audience - meaning there was a real-life Dancing Queen in attendance.

The concert has been in the works since 2016 and features ground-breaking new technology that recreates the sight of Abba in their 1970s prime, playing hits like SOS, Voulez-Vous and Lay All Your Love On me.

To create the spectacle, the band performed in motion capture suits for five weeks, with 160 cameras scanning their body movements and facial expressions.

Those became reference points for hundreds of animators and visual effects artists to create avatars of the band in their heyday. 

Affectionately known as "Abba-tars", the characters are not 3D holograms - as everyone involved in the production is at pains to point out.

"I don't think any hologram shows have been successful,"producer Baillie Walsh told Dazed magazine last year. "After five minutes, I don't think they're that interesting."

Instead, the characters appear on a massive, 65million pixel screen, with lights and other effects blurring the boundaries between the digital elements and the "real world" in the arena.

"We want to pull on the emotions," producer Svana Gisla told the BBC.

"So if you come out of here and feel like you've seen a visual spectacle, we will have failed. If you come out of here and you've laughed and you've cried and you can't wait to go back, that's what we want."

Amazingly, they've pulled it off. The images might be 2D, but impressive lighting effects and back projections provide a crucial depth of field - creating the illusion that the band are really in the room with you. 

I was cynical about the technology ahead of the show, but the effect is mystifyingly realistic. It needs to be seen to be believed.

As the concert begins, the quartet "emerge" from under the stage on risers, before launching into the spooky electric psychodrama of The Visitors - the title track of their penultimate album, from 1981.

A photo-realistic Agnetha and Frida resemble peacocks in red and blue-winged catsuits, twirling around the stage while Benny and Bjorn vamp on the piano and guitar.

Although the band members were in their 70s when the motion-capture footage was filmed, the Royal Ballet's resident choreographer Wayne McGregor helped them recreate the movement of their younger selves.

Agnetha and Frida's home-spun dance routines, which could charitably be described as "extravagant walking", are both instantly recognisable and achingly nostalgic.

While the virtual Abba are restricted to the centre of the stage, a spectacular floor-to-ceiling light show adds a propulsive dynamism to proceedings. The band perform Chiquitita against an eclipsing sun, and are surrounded by pulsing laser beams during Gimme! Gimme! Gimme! (A Man After Midnight).

As long as you keep your eyes away from the big screens, where the avatars assume an unfortunate "uncanny valley" effect, you feel like you've been transported back to the band's last UK concert, in London's Wembley Arena, 42 years ago.

Wisely, the production acknowledges all the digital trickery, with Benny's avatar declaring: "To be or not to be, that is no longer the question," during an early interlude.

"This is the real me," he continues, to laughter from the audience. "I just look very good for my age."

The music is enhanced by a precision-drilled, 10-piece live band, who race through an eclectic set of Abba songs that mixes classics like Mamma Mia and Thank You For The Music with unexpected deep cuts such as Eagle and the tender When All Is Said And Done.

The two new songs they released last year, Don't Shut Me Down and I Still Have Faith In You, slot into the setlist seamlessly; while the footage of the band winning the Eurovision Song Contest in 1974 elicits an audible "aaaahh" from the audience during Waterloo.

But there are some surprising gaps, with The Name Of The Game, Super Trouper and Money Money Money among the songs consigned to the dustbin of history.

It seems likely, however, that Abba recorded more songs than currently appear in the show, to encourage repeat visits.

But no matter what happens next, the opening night audience of celebrities and hardcore fan club members was rapt.

"That was frickin' incredible," pop star Zara Larsson told the BBC after the show. "I cried four times. I didn't know I was going to feel like that."

"It felt like I was experiencing them in their prime and it was very emotional."

"It was an extra-terrestrial, extraordinary, exhibition of fabulousness," agreed choreographer Les Child. "They've done an amazing job."

"I felt like I spent the evening with Abba" added Hanna Rossman, who had travelled from Berlin for the show. "And the absolute highlight was that they came on stage at the end."

The quartet didn't speak during their brief appearance on stage - but, by then, the concert had said everything they needed to say.

The show, which takes place in a purpose-built arena in east London, is currently due to run until December 2022. Then, in true Swedish style, the venue can be collapsed into a flat-pack and taken on the road.

It's the perfect solution for a band who swore never to tour again after they dissolved in 1982 - even turning down a $1bn offer to play 100 shows at the turn of the Millennium.

But the new technology, pioneered by Star Wars VFX company Industrial Light & Magic, tempted them back into the live arena.

"Being able to be on stage and perform for an hour or two, while being home walking the dog or making a carbonara? That's how it all started. We were intrigued by that," Benny Andersson told the BBC last year.

"And the vision of creating something spectacular which no-one had ever seen before," added Bjorn Ulvaeus. "I think, after we open [this show] there might be one or two who want to do the same, while they're still alive."

So, could the concert be replicated in purpose-built Abba Arenas around the world?

"It all depends," said Benny. "We'll see how it goes in London. Are people really interested in coming to see this?

"If they are, yes, it'll continue. If they're not coming, then we just close."

On tonight's evidence, the show will go on.