Prince William was "furious" about the way in which Queen Elizabeth II's private secretary was forced out following a power struggle with the then Prince of Wales and the Duke of York, a new book has claimed.
Lord Geidt, formerly Sir Christopher Geidt, was ousted from the Royal Household in 2017 after falling foul of the two brothers in his bid to modernise the monarchy.
He had acted as a mentor to Prince William for years - ensuring he spent plenty of time with the late Queen on both official and personal business, giving him the chance to learn the ropes and observe how she got things done.
One source said: "William was furious. He spoke to his grandmother and father.
"He felt Christopher had worked to modernise the institution and bring it closer together. He was concerned about the way it had been handled, and how Christopher had been treated."
The book, Courtiers: The Hidden Power Behind the Crown by Valentine Low, also revealed that Margaret Thatcher needed a whisky after her weekly audience with the late Queen.
Lord Butler, Thatcher's former private secretary, described how the then prime minister would join him and Sir Philip Moore, the Queen's assistant private secretary, after the meeting for a debrief.
"Margaret Thatcher was notoriously quite tense with the Queen," said Lord Butler. "She needed a whisky afterwards."
Lord Geidt had been the late Queen's private secretary for 10 years.
His tenure came to an abrupt end amid differences over how to manage the transition of power between the late monarch and her eldest son.
When told he had been let go, the new Prince of Wales is said to have gone to see Earl Peel, the Lord Chamberlain, to give him a piece of his mind.
"He was really angry about it, not necessarily because it was the wrong decision," said a source.
"He just thought it was handled very unkindly for a man who was a pillar of the institution of the monarchy, but had also played an incredibly important role when the coalition government had been formed."
Lord Geidt 'bruised' after power struggle
Lord Geidt had fallen foul of the Duke of York when, during the Diamond Jubilee celebrations in 2012, he supported Charles's bid to present a slimmed down version of the monarchy on the Buckingham Palace balcony.
Rather than the traditional scenes of the wider Royal family cramming onto the balcony for the flypast, just the late Queen and the then Prince of Wales and Duchess of Cornwall, Duke and Duchess of Cambridge and Prince Harry stepped out before the crowd.
It was a deliberate act to convey a message about who really mattered in the Royal family - but for the Duke of York, Low claimed it was "like a dagger to his heart and he hasn't got over it".
In 2014, Lord Geidt's attempts to create a streamlined royal communications operation, bringing all households under one umbrella, did not go down with Charles III, who is alleged to have felt that his power base was being undermined.
In May 2017, when the Duke of Edinburgh retired from public life, he told staff that all members of the Royal family and their households should unite and act collectively in support of Queen Elizabeth.
That message also proved problematic, the book claimed, with fears abounding about the merging of households. Younger royals were also left with the impression they were being told to do less of their own campaigning work.
A few weeks later, the late Queen's eldest two boys are said to have joined forces to tell the monarch: "Geidt has got to go."
Lord Geidt, who went on to work as an ethics adviser to Boris Johnson before resigning earlier this year, was said to have been "very bruised" by the brutal manner in which his downfall was orchestrated.