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".
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So-called Islamic State said they were behind the attack that targeted people receiving their national identification cards.