Prince William is deeply unhappy over his younger brother's decision to issue an unprecedented public statement about his new girlfriend, Meghan Markle.
The future king is understood to be concerned that "hot-headed" Prince Harry officially confirmed his relationship with the 35-year-old American actress by releasing an emotional tirade against the media and online commentators, sources said.
William, 34, and Harry, 32, are very close, living yards from each other at Kensington Palace and sharing both an office and a charitable foundation.
But despite his support for Harry, William is fiercely protective of his family's privacy and believes his more open brother made a mistake in wearing his heart on his sleeve.
"He thinks Harry made a mistake in confirming the relationship and releasing such an emotional statement so soon [into it]," one well-placed Palace source told the Daily Mail.
"He feels that the statement offered up too much information and only served to fuel speculation about how serious Harry is about Meghan. It could also put him in a difficult position in the future regardless of whether the relationship leads to something or not.
"William knows that his brother can be hot-headed and understands that it is something he felt very strongly about. Privately, he shares a lot of the sentiments he and Meghan expressed as, make no mistake, she was very involved in the statement too.
"But he believes it was wrong to make his feelings so public."
Royal sources said yesterday that William was made aware of the statement before Harry put it out. A Kensington Palace spokesman added the elder prince 'understood' Harry's concerns. They said: "The Duke of Cambridge absolutely understands the situation concerning privacy and supports the need for Prince Harry to support those closest to him."
Over the years the brothers have issued only a handful of public statements in connection with their personal lives.
Some have concerned their late mother, Diana, Princess of Wales, including when an Italian magazine printed a photograph in 2006 of the royal as she lay dying in the wreckage of her car.
They also accused her former butler, Paul Burrell, of a "cold and overt betrayal" after he wrote a tell-all book about his time working for her.
William spoke out after topless photographs of his wife, the Duchess of Cambridge, were published in a French magazine in 2012, describing the incident as a "grotesque" invasion of their privacy.
And more recently the prince issued a warning to paparazzi photographers over their attempts to secure pictures of his children, Prince George and Princess Charlotte.
But when it comes to their personal relationships the princes have been more circumspect, knowing whatever they say will only fuel public interest.
William spoke once about the level of paparazzi interest in Kate Middleton, then his girlfriend, and some years into their relationship when she was being followed daily by dozens of photographers amid rumours the pair were getting engaged.
"William is of the opinion that you need to keep your powder dry on these sorts of things and they, as a family, should only resort to making public statements when there is no other option left," said the source.
"He just doesn't feel this was a big enough issue to make a fuss about."
Harry is currently on a two-week tour of the Caribbean on behalf of the Queen. Yesterday he took part in a cricket match in St Lucia. He will return to London for engagements in early December but is then expected to jet off to see Miss Markle before spending Christmas with the Royal Family at Sandringham.