Prince Harry has asked his elder brother the Duke of Cambridge to be his best man when he marries US actress Meghan Markle next month, his office says.

The announcement goes against traditional royal protocol under which the groom is normally accompanied by a “supporter”, not a best man.

The tradition was previously discarded when Harry stood as William’s best man at his wedding to wife Kate 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,” Kensington Palace announced on Twitter, sharing an image of the pair as children.

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. pic.twitter.com/mQ0eh7Q0pR

— The Prince and Princess of Wales (@KensingtonRoyal) April 26, 2018

William later said he was “thrilled and delighted” to be asked to be best man, but cryptically warned “revenge is sweet”.

William and Harry briefly spoke to reporters as they made an unscheduled appearance at the opening of a sports centre in London on Friday (Thursday local time).

Earlier this week William and Kate, whose official titles are the Duke and Duchess of Cambridge, celebrated the arrival of their third child. Royal watchers are still awaiting the announcement of the new prince’s name.

William refused to reveal the name when asked by reporters for a clue on Friday, saying only: “I’m working on it.”

The best man role means William will miss English soccer’s FA Cup final as it clashes with the wedding, media reported.

William, as President of the Football Association, usually awards the trophy to the winners.

Harry and Meghan have invited 600 guests to their wedding and a lunchtime reception afterwards at Windsor with another 200 attending a private evening event at nearby Frogmore House.

No political leaders are among the invitees and the wedding guests will be made up of people who have an existing direct relationship with the groom or the bride.

However, the couple have invited 1200 members of the public into the grounds of Windsor Castle, Queen Elizabeth’s home west of London, as part of the celebrations.

Details of bridesmaids and who else will make up the wedding party have yet to be released.

-with AAP