The message was also accompanied by a picture of a smiling Harry and an emoji of a birthday cake.

Around an hour later, the same post was shared by the Prince and Princess of Waleses’ X account along with the message: “Wishing a Happy 40th Birthday to The Duke of Sussex!”

Some will see the post as a surprise move given the unresolved tensions between the brothers, which has shown no sign of abating.

But according to royal biographer and editor-in-chief of Majesty Magazine Ingrid Seward, there was a big reason for the message.

She told the Mirror: “King Charles would have celebrated his son’s 40th from a distance. Milestone birthdays are important to the royals. Whatever upsets Harry has caused would have been forgotten for the day.

“Even Prince William wished his brother Happy Birthday – even if it was through clenched teeth. He doesn’t trust him, but together with his family, he wished him the best on his milestone birthday.”