Geri Halliwell says she was holding back the tears when she watched the workshops of the new musical.
Mel C added that the rehearsals gave the girls "goosebumps" and it was a "lovely thing"
The girls said the musical supports Girl Power's longevity. "Who would have thought the message is still standing 16 years later".
More top news
The returns of Benedict Cumberbatch's detective and the X Factor judge were among 2014's most tweeted TV moments in a billion conversations.
A family has issued an appeal to help find their missing pet turkey - in case he gets gobbled up as someone's Christmas dinner.
The winner of X-Factor Ben Haenow has revealed how keen he is to have the Christmas number one.