The 32-year-old Prince roared with laughter when Master Of Ceremonies Richard Birtchnell mistakenly announced him as his 93-year-old grandfather.William was at the Institute to present Bill Gates' wife Melinda with the Chatham House Prize, in recognition of her work to end poverty and empower women.
Prince William laughed as he was accidently introduced as the Duke of Edinburgh
As he presented the philanthropist with her prize of a crystal statuette and a scroll signed by the Queen, William praised her for her hard work with the Gates Foundation.
"Civil society, whether in the form of business, academic institutions, NGOs or philanthropic organisations, play a critical role in shaping international relations and in working for a fairer, more peaceful and healthier world," he said. "No one could provide us with a better example of the impact such organisations can have, when led with vision, commitment and unstinting generosity, than this year's Chatham House Prize winner, Melinda Gates.
The Prince was presenting Melinda Gates with the Chatham House Prize
"With her equally remarkable husband Bill, Melinda set up the Gates Foundation to change the world," he added. "And change it she has, transforming the lives of women, the poor, the sick and the disadvantaged in Africa and elsewhere. What a wonderful example it is to others - how fitting it is to add Melinda Gates's name to the Chatham House Prize list."