LONDON - Britain's Prince Harry has asked his elder brother Prince William to be best man at his wedding to U.S. actress Meghan Markle next month, his office said on Thursday.

Harry was William's best man at his own glittering wedding to wife Kate Middleton in 2011 and has now requested his sibling repays the favour when he marries Markle at Windsor Castle in just over three weeks' time.

"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 said in a statement.

The role means William, whose official title is the Duke of Cambridge, 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 Markle 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 1,200 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 Kensington Palace issuing updates every few days about the wedding which has drawn huge worldwide media interest.

The British royals have already had much to celebrate this week after the birth on Monday of William and Kate's third child. The newborn prince, who follows siblings George and Charlotte, has yet to be named and pushes Harry down a place to sixth in line to the throne.

Meanwhile Markle made her final TV appearance as an actress on Wednesday night, marrying long-time love interest Mike Ross in the U.S. legal drama "Suits".

(Reporting by Michael Holden; editing by Kate Holton) ((michael.holden@thomsonreuters.com; +44 207 542 3213; Reuters Messaging: michael.holden.thomsonreuters.com@reuters.net))