It was in 1996 that the Spice Girls filmed the video for their first single, Wannabe, as unknowns at London's St Pancras Hotel. On Tuesday they returned, and proved themselves still capable of creating a stir as they launched the next phase of "girl power": a West End musical based on their songs from the woman behind Mamma Mia!

All five turned up, reuniting for the first time in four years; Scary Spice at one end, saying the wrong things; Posh Spice at the other, not saying the wrong things.

In the middle was Ginger Spice. "Wow," said Geri Halliwell. "Sixteen years ago I couldn't have imagined when we were writing the lyric 'zigazig ah' it would end up in a West End show."

"We're introducing a whole new generation to girl power," said Victoria Beckham. "What we do individually is really empower women."

Mel C admitted: "It just feels so surreal and we're so excited to finally be here. We've seen a lot of the workshops of the show and it is incredible. It's better than we could ever have hoped for."

"They sing it better than us," added Mel B, probably joking. Asked about the Spice Girls legacy, she said simply: "All my bitches are great."

The group were in London for the launch of Viva Forever!, a musical written and produced by Jennifer Saunders and Judy Craymer respectively, which opens at the Piccadilly Theatre in December. The show follows the "juke box musical'' formula set by shows such as Mamma Mia!, which features Abba songs, and We Will Rock You, with its Queen soundtrack.

Unusually, it was the writer who approached the producer. "I persuaded Judy to take me," Saunders told the Guardian. "I heard the project was going and I said to my agent: 'Tell Judy I want to do it'. It's because I love the Spice Girls so much, my daughters grew up with the Spice Girls and we used to go and see them and I knew them a little bit because of Comic Relief.

"When my girls were little teenagers, eight-12-year-olds, the Spice Girls gave them such a lot of fun and that is the sort of thing you don't get with a lot of girl groups nowadays – that idea of anarchic fun and you can just actually kick about a bit, you can say what you like a bit.

"There's an irreverence, but also an ability to speak to kids and grown-ups and for it not to be a sexualised, threatening experience."

Saunders insisted she'd been interviewed but Craymer joked: "She completely bluffed her way into the job. I had a working title, Viva Forever, because it's one of my favourite songs and it's the most romantic song – Jennifer had heard that and she bluffed her way in to the office and said I've got this idea: 'It's about a girl called Viva' and that was it she was off."

Craymer said the Spice Girls defined an era. "They gave confidence to women. They wanted to create something which crossed generations."

It was decided early on, with the Spice Girls' agreement, that it would not be a tribute or biographical show. Instead the same formula will be employed that has helped Mamma Mia! become one of the most profitable shows on the planet – an emotional story about ordinary people who just happen to break into song when the moment seems appropriate. From the workshop video that was shown at the launch the story will be about a red wine-liking mother who lives on a houseboat with Viva, her spirited daughter. Viva is in a group with three friends and they try out for a TV talent show. They do well, but TV bosses suggest Viva goes it alone as a performer.

Craymer said there had been a lot of listening to Spice Girls songs which explore friendship and optimism and fame. Saunders added: "You could play Who Do You Think You Are now and it's relevant to everyone now with Facebook and Twitter and TV talent shows."

How well the reputation of talent shows come out remains to be seen. Craymer said the central story was "emotional and heart warming" and there would be comedy from the fact that "on TV talent shows the judges see themselves as the Gods of Olympus."

Saunders admitted to being a keen watcher of such shows but said she had gone off X Factor. "I thought The Voice was much better. I liked the judges. X Factor is so formulaic now."

Main photograph: Dave Hogan/Getty