The Duke of Sussex will celebrate the milestone on September 15, and it is thought his grandmother, the late Queen Elizabeth, started collecting a huge trust fund for him when he was aged 10.
At the time, the Queen Mother, who was then 94, apparently put aside a tax-free 19 million pounds ($A37 million) to leave to her great-grandchildren, according to The Times.
The newspaper added the inheritances were divided into two instalments -one to be given to Princes William and Harry when they turned 21 and another on their 40th birthdays.
A former palace aide told The Times: "There was a trust fund set up at the time.
"It was a way in which the Queen Mother could set aside money for when her great-grandchildren were older and a way of passing a slice of her estate down in a tax-efficient way.
Prince Harry will celebrate his 40th birthday on September 15. (AP PHOTO)
"It was a way in which some of her estate could be ring-fenced for them."
Sources added Harry may have been left more money than his older brother William, 42, in the inheritance.
Insiders say it is due to the fact the Queen Mother assumed William would get a larger fortune from the Duchy of Cornwall as the future heir.
William became Duke of Cornwall when he was made Prince of Wales after King Charles' coronation and is estimated to have got a 23.6 million pound fortune by inheriting the estate.
When the Queen Mother died in 2002 aged 101, a statement from Buckingham Palace said: "Queen Elizabeth The Queen Mother has bequeathed her entire estate (which mainly comprises the contents of her houses) to The Queen.
"In her will, she asked The Queen to make certain bequests to members of her staff, and these bequests will be subject to inheritance tax in the normal way.
"The Queen has decided that the most important of Queen Elizabeth's pictures and works of art should be transferred to the Royal Collection."
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