The 35-year-old royal is ''honoured'' to have been asked to perform the role at his younger brother's nuptials to Meghan Markle at St George's Chapel in Windsor on May 19, especially as his 33-year-old sibling had the same position when he married Duchess Catherine in 2011.
Kensington Palace announced in a statement: ''Prince Harry has asked his brother The Duke of Cambridge to be his Best Man at his wedding to Ms Meghan Markle.
''Prince Harry served as best man to The Duke of Cambridge at his wedding to Miss Catherine Middleton in 2011.
''The Duke of Cambridge is honoured to have been asked, and is very much looking forward to supporting his brother at St George's Chapel, Windsor on May 19th.''
To mark the announcement, the Kensington Palace Twitter account shared a number of pictures of the brothers together, including one of them as children wearing police helmets, and another of them having a lightsaber fight at the studios where 'Star Wars' is filmed.
It hasn't yet been announced who will be Meghan's bridesmaids and page boys, but William and Catherine's oldest children, Prince George, four, and Princess Charlotte, two, are expected to form part of the wedding party.
The couple also became parents to another baby boy earlier this week, though his name hasn't yet been revealed.
Around 600 guests have been invited to the wedding, including members of the public who will be able to wait in the chapel grounds to glimpse the happy couple up close.
Actress Priyanka Chopra and fashion designer Victoria Beckham have both hinted they're on the list, while Harry's former girlfriends Chelsy Davy and Cressida Bonas are also rumoured to have been invited.
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