Prince Harry turned 37 on September 15 and was sent the typical birthday tributes from members of the royal family.
Prince William and Kate Middleton shared a very brief message to mark the occasion from their Twitter and Instagram account with the message simply reading, "Happy Birthday Prince Harry!"
The message was paired with a snap of Harry and a balloon emoji.
Royal expert Duncan Larcombe told The Sun, William and Kate's "curt" message "only serves as proof the ice between the brothers has still not thawed".
"Short of ignoring the Duke of Sussex's 37th landmark completely, their social media post was the bare minimum from a couple who once welcomed Harry as their official Royal gooseberry," he explained.
"If anything, this year's birthday wishes from the Cambridges are an indication of just how far relations between the two warring clans have deteriorated over the past 12 months."
The pair has been 'feuding' since Harry began dating Meghan Markle and soon after proposed with William warning his younger brother that he was perhaps moving too fast.
In his bombshell interview with Oprah Winfrey earlier this year, Harry claimed William was "trapped" but didn't even realise it, which is believed to have angered the Duke of Cambridge further.
Last year for Harry's birthday, the Cambridges shared a group photo of the three of them participating in a running race with the caption, "Wishing a very happy birthday to Prince Harry today!"
In 2019, they shared a photo of the brothers smiling at Harry's wedding in 2018.
Larcombe added that the pair had attended a number of events together this year, one of which was the unveiling of a statue in their mother Princess Diana's honour, and was a little surprised they didn't choose a photo from that event.
"That would have sent a message of unity and the shared common bond between siblings who were once as thick as thieves," he said.
Earlier this year, Robert Lacey, author of Battle of Brothers, told Daily Mail that a source shared with him, "The rage and anger between those two has grown so incredibly deep."
"Too many harsh and wounding things have been said."
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