The King is to become the first head of state to reveal their personal tax bill to help enhance transparency around royal finances.

Charles will publish his financial details as his royal household seeks to increase "clarity and accessibility" around the monarchy's finances.

The decision by the King is in contrast to the Prince William who has not disclosed the tax he has paid since becoming prince of Wales and heir to the throne.

"While this is the first time a monarch has shared this personal tax information, you may recall it was similarly released by His Majesty when he was prince of Wales," a Buckingham Palace spokesperson said.

"The decision to do so as sovereign has come at the express wish of the King himself, as part of the adaptations carried across since accession."

The King's private sources of income could include money from investments or trading profits, funds generated by his private estates of Balmoral and Sandringham and private savings.

Charles pays tax on all his private income, and capital gains tax on relevant elements of his assets, as laid out in the Memorandum of Understanding on Royal Taxation 2023, agreed by the government.

Later this week the King's total personal tax information for the 2024-25 financial year will be published, alongside other reports on royal finances.

His 2025-26 tax details will be released next year when their audit has been completed.

William receives an income from the Duchy of Cornwall, a billion-pound estate featuring The Oval cricket ground and Dartmoor Prison, providing the heir to the throne with funds independent of the monarch.

The prince received nearly £23 million ($A43 million) last financial year from the duchy and voluntarily pays the highest rate of income tax, once official costs have been deducted, but the amount he pays in tax is not disclosed.

Accounts detailing the Sovereign Grant, which funds the official duties of the royal family, will be published at a press briefing this week alongside a separate extensive new royal household report on royal finances.

Figures from last year showed the Sovereign Grant stood at £86.3 million ($A162.9 million), comprising £51.8 million ($A97.8 million) for core elements like travel and payroll costs and £34.5 million ($A65.1 million) to fund the Buckingham Palace Reservicing Program.

"Our aim is to explain all elements of royal finances in a way that further enhances clarity and accessibility," a Buckingham Palace spokesperson said.

"To put it simply: we continue to modernise and evolve."