The Duke and Duchess of Cambridge arrived for the first ever Earthshot Prize Awards launched by Prince William on Sunday.
Kate Middleton, Emma Watson and others were to host the ceremony and present awards.
Queen Elizabeth II's grandson Prince William will bestow his inaugural "Earthshot" environmental prize at a gala ceremony on Sunday, after a buildup marked by royal displeasure at world leaders' inaction on climate change.
At the televised event -- featuring the renowned naturalist David Attenborough and performances by Coldplay, Ed Sheeran and others -- five international winners will each receive £1 million ($1.4 million, 1.2 million euros).