Queen Elizabeth fears that Britain will be shocked by the different style of monarchy Prince Charles is planning, according to a new biography of her heir apparent.

'Charles: The Heart of a King' claims that the 66-year-old prince's passion for embracing sometimes unusual causes has prompted disquiet at Buckingham Palace, where the 88-year-old queen is set to overtake Queen Victoria in September as Britain's longest reigning monarch.

When his turn finally comes to ascend the throne, Charles plans a new model of kingship that would create a smaller monarchy and open up royal residences to the public, according to the unauthorised biography.

Charles's father, the Duke of Edinburgh, is among his harshest critics, the book claims, and believes the prince to be guilty of "selfish behaviour" in putting his "cerebral passions" before his royal duties.

Charles himself is quoted by the author as saying: "I only take on the most difficult challenges... I want to raise aspirations and re-create hope from hopelessness and health from deprivation."

A spokeswoman for Buckingham Palace declined to comment on the book, but a spokeswoman for the Prince of Wales released a detailed statement, saying the author Catherine Mayer had not been given exclusive access to the prince and emphasising that her book was not an official biography.

The eldest son of Queen Elizabeth and groomed from birth to one day be king, Charles has taken on the responsibilities of public life but found himself for years eclipsed by Princess Diana, his first wife who died in a Paris car crash in 1997.

In 2005 Charles married Camilla Parker Bowles, who will one day sit alongside him as queen according to the current assumption in Buckingham Palace, Mayer wrote.