Diana, Princess of Wales, left a net estate of £12,966,022 after inheritance tax of £8,502,330 was deducted following her death in 1997.
With shrewd investment, the fund is likely to have grown to around £20 million since then, of which the Duke is entitled to a half share.
In her will, Diana stipulated that her estate, which comprised stocks and shares, jewellery, cash from her £17 million divorce settlement, dresses and other personal belongings, should be held in trust to be shared equally between her sons.
The will stated that both William and Harry would be entitled to their entire share of the capital on reaching the age of 25, but after her death the executors changed the terms of the will to raise the inheritance age to 30.
Since the age of 25, the Duke has been entitled to income from interest on the capital, as well as earning around £40,000 a year as a pilot. He can also draw on money, believed to be about £2 million, left in trust by his great-grandmother Queen Elizabeth.
The will does, however, allow him to vary its terms to give his brother Prince Harry the lion's share of the money, which he might choose to do to reflect his guaranteed future income from the Duchy of Cornwall when he becomes Prince of Wales.