Britain's Prince William wished his estranged younger brother Harry a happy birthday, as the younger son of King Charles III turned 40 on Sunday.

Harry's ties with his family have been increasingly fraught since he and wife Meghan Markle quit royal life and moved to California in 2020.

William and Harry used to be close -- a bond that was forged with the death of their mother Princess Diana in 1997. But according to British media reports, they have not spoken to each other in two years.

"Wishing a Happy 40th Birthday to The Duke of Sussex!", read a post on William and wife Catherine's official X account, referring to Harry's royal title.

It shared an earlier message on the royal family's X account which read: "Wishing The Duke of Sussex a very happy 40th birthday today!".

That post was accompanied by a photo of a smiling Harry and an emoji of a birthday cake.

UK media said the post from the royal family account marked the first public birthday message for Harry since 2021.

Harry, who has two young children, said on Friday that he was "excited about 40", despite his strained family ties.

"I was anxious about 30, I'm excited about 40," he told the BBC in a statement, adding that becoming a father had given him a "fresh perspective on life".

"Being a dad is one of life's greatest joys and has only made me more driven and more committed to making this world a better place," he added.

William and Harry were seen together at the late Queen Elizabeth II's funeral in September 2022, but have pointedly kept their distance at subsequent events, including at their father's coronation in May last year.

Harry's autobiography "Spare" published in January 2023 described William as his "beloved brother" but also his "arch-nemesis".

Royal experts have said they do not see a reconciliation on the horizon.