Prince Harry's inheritance payday on his 40th birthday will surpass Prince William's share as the "spare," according to an expert.

Prince Harry is expected to have a very happy birthday this year.

The Duke of Sussex is expected to receive a substantial amount of money on his 40th birthday, which falls on September 15th, as a result of arrangements made by his late great-grandmother.

The Queen Mother died in 2002 at the age of 101.

According to Christopher Andersen, author of "The King," the Queen Mother invested two-thirds of her estimated $120 million fortune into a trust fund for her great-grandchildren.

"When Prince William became Prince of Wales, he inherited the Duchy of Cornwall, which now has assets worth over $1 billion. The Queen Mother may have left Harry a bit more in the division of things, but the value of the trust has only increased, along with Harry's share."

According to the U.K.'s Mirror, Harry will receive a larger share of the inheritance than William, amounting to $8.5 million.

According to Kinsey Schofield, host of the "To Di For Daily" podcast, Prince Harry will receive more money than Prince William because he is the spare. The Queen Mother made this decision with the understanding that Harry would have fewer opportunities and responsibilities.

"Prince William likely quietly inherited his share two summers ago when he turned 40, as the family knew about the arrangement for decades and there were no tensions."

According to Andersen, the reason trust doesn't develop until age 40 is because of something.

"Prince Harry is set to receive more money than Prince William because he is the spare." — Kinsey Schofield, host of the "To Di For Daily" podcast

Andersen stated that for a woman who almost reached 102 years old, 40 must have seemed like the ideal age to receive some extra money - young enough to enjoy it but also old enough, and hopefully wise enough, not to squander it.

"It's likely that the princes' great-granny would have disowned Harry entirely if she had known about the trouble he caused the family."

In 2020, the U.K. Times reported that the Queen Mother bequeathed a portion of her wealth to Harry, who received it through a trust fund intended for her grandchildren.

In 2002, Queen Elizabeth II was reported to have avoided paying an inheritance tax of nearly £20 million ($25 million) on her mother's legacy, which increased the value of her grandchildren's fortune. Additionally, both William and Harry paid £8.4 million ($10 million) in inheritance taxes on the £21 million ($27 million) estate of their late mother, Princess Diana.

Harry, 39, must pay taxes on any future commercial earnings, the outlet stated.

On his birthday, Harry will receive a larger payout than his brother, but William still has some money.

The Duchy of Cornwall released its Integrated Annual Report for the 2023-2024 financial year, marking William's first full year as the Duke of Cornwall. As the heir to the British throne, William inherited the estate and title from his father, King Charles III, upon the latter's ascension to the throne at the age of 75.

In 2022, England's longest-reigning monarch, Queen Elizabeth, passed away at the age of 96.

The Duchy of Cornwall estate yielded William, 42, more than $30 million in his first year's income.

William, as a member of the royal family, does not receive a typical income, People magazine reported. Instead, his annual expenses are mostly covered by the Duchy of Cornwall, which was founded by King Edward III in 1337 to provide funds for the heir.

The estate, valued at over $1 billion, covers 130,000 acres across 23 counties in England and Wales. The estate comprises land, farms, houses, and other assets to support the heir and their family. It is reported that William pays income tax on the entire income he receives after deducting household expenses, which have not been specified.

According to British broadcaster and photographer Helena Chard, Prince Harry has been fortunate in terms of trust due to his perceived future financial prospects, which differ from those of Prince William.

Prince William has a private estate, the Duchy of Cornwall, which he benefits from. However, Prince Harry will not receive any private benefits from this estate. The late Queen Mother set up a trust fund in 1994 to ensure that her great-grandchildren would not have unfettered access to their inheritance and would be required to reach a certain age before receiving it.

"Prince Harry's decision to step down from full-time royal duties likely came as a surprise to the late Queen Mother, who would have expected him to remain Prince William's right-hand man. The inheritance split may have caused mixed feelings, but the royal family will be relieved that Harry has enough funds to keep him out of trouble."

Although William was predestined to become king from birth, Harry, who is fifth in line to the throne after his brother and William's three children, has frequently seemed to struggle with the more unclear role of "spare."

He selected "Spare" as the title of his 2023 memoir, which detailed a lifetime of sibling disputes, including William's preference for the larger bedroom at Balmoral Castle and Harry's heated argument with William over a necklace.

Schofield stated that the Queen Mother consistently preferred Prince William over his brother, Prince Harry. She would frequently spend time with William as a child and exclude Harry from her interactions. This favoritism caused Princess Diana to become increasingly agitated.

An expert believes that Prince Harry and Prince William need an intervention to end their feud.

"The Queen Mother had a significant impact on King Charles's life as a young boy and played a role in mentoring him for his future role as a sovereign. Similarly, she desired to have a similar relationship with Prince William and imparted to him knowledge about his elders and responsibilities."

Harry claimed that he and Meghan were portrayed as villains by the British media while William and Kate were viewed as virtuous, and he accused palace officials of lying to protect his brother.

The reasons for the Sussexes' decision to quit royal duties and move to the U.S. in 2020 were the media's racist treatment of Meghan, who is biracial, and a lack of support from the palace.

In an explosive interview with Oprah Winfrey in 2021, the couple claimed that the royal family was indifferent to Meghan's mental health struggles and suggested that an unnamed family member made a racist comment to Harry before the birth of their son, Archie. Harry also described his relationship with William as "space at the moment."

Harry lives in California with his wife and their two young children.

The Associated Press contributed to this report.