Royal author Duncan Larcombe made the claim as he discussed the Duchess of Cambridge's upcoming 40th birthday. Mr Larcombe said that previously the Duke of Sussex would have been "instrumental" in any party for Kate's milestone.

       However, the royal expert claimed "those days are long in the past" as he highlighted Prince William and Harry's rift.

       Mr Larcombe told OK! magazine: "It will be very interesting to see what messages Harry and Meghan send Kate.

       "This is the problem, as the rift continues, as the war between the boys seems to show no sign of finishing, days like Kate's 40th birthday milestone immediately become events that can be contrasted with how it used to be.

       "Kate was, as Harry said, the sister he never had and clearly that is not the case anymore.

       "There was a time when Harry would have been instrumental in any celebrations or any fun that the couple were having, but those days are long in the past."

       Kate turns 40 this Sunday and is expected to mark the day privately.

       Meanwhile, Harry and Meghan Markle are living in California after quitting royal duties.

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       Kate is likely to celebrate her birthday with William and their three children Prince George, Princess Charlotte and Prince Louis.

       It comes after the Cambridges spent Christmas at Anmer Hall in Norfolk.

       They were joined by some members of the Middleton family for the festive day.

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       If Kate has a party for her birthday, her parents Carole and Michael may help organise the bash.

       The Middletons run their Party Pieces family business, which sells items from balloons to personalised cakes and banners.

       And Kate's is not the only family birthday this month.

       Her mother Carole celebrates her big day on January 31.

       In a post on the Party Pieces Instagram account on Wednesday, Carole hinted she would be having a party.

       She said: "It’s no surprise I‘ve learned to love January; it’s the start of a new year and time to get healthy habits in place and birthday month for me, so there is every good reason to celebrate with loved ones!"

       The Middletons are known to be a close-knit family and they recently supported Kate at a carol concert she hosted in December at Westminster Abbey.