The Duke and Duchess of Cambridge attended a meeting of the Thirty Club, at Claridge's in London.
The club members are some of the most powerful men and women in advertising and the media and meet for monthly dinners to discuss the issues of the day.
Their meetings feature a guest speaker and this month it was Prince William with his wife joining him for support.
A St James's Palace spokesman said William had decided to attend in honour of the Queen's jubilee and his own 30th birthday next month.
They said: "The Duke has been invited for around nine years now and thought that it was right to accept the invitation this year because of the Queen's Diamond Jubilee.
"It also seemed rather apt as it his 30th birthday next month.
"Other senior members of the Royal Family also suggested that it was an interesting event to attend."
The Duchess looked extremely glamourous in a full length, fitted cream evening gown with a daring thigh-high split.
She accessorised with a matching clutch bag, and a £450 pair of metallic, strapped high heels by designer Jimmy Choo.
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