The Duke of Cambridge will be Prince Harry's best man, Kensington Palace has confirmed.
William is "honoured" to have been asked by his younger brother to perform the special role, and is looking forward to the task.
Underneath the public banter, where they regularly poke fun at each other, the two royal brothers are very close - and Harry famously served as best man at his brother's 2011 wedding.
Kensington Palace said in a statement: "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."
The Duke of Cambridge, 35, and thirty-three-year-old Prince Harry have a close bond.
Their shared memories of childhood and the support they offer each other have been invaluable in adulthood.
Growing up together, they jointly experienced the scrutiny that comes with royal life, and now frequently make jokes about each other's hair at royal appearances.
Both have served in the military, having faced the rigours of training at Sandhurst, they play polo together and live close to one another.
When Harry turned 21, he described William as the one person on the planet with whom he could talk about everything.
"It's amazing how close we've become," Harry said. "I mean, ever since our mother died, obviously we were close, but he is the one person on this earth who I can actually really ... we can talk about anything.
"We understand each other and we give each other support."