During a walkabout in Brisbane, Prince Charles bristled to an Australian journalist: ‘Don’t believe all that crap.’ The ‘crap’ the then 69-year-old Prince of Wales was referring to in 2018 came from my bestselling biography, Rebel Prince, published that year.

Filled with eyewitness accounts about a vain, out-of-touch, petulant and financially reckless prince, the book recounted Charles’s habit of dispatching a large truck to a host’s home days before he arrived.

Not only did his staff install the eccentric prince’s own bed, radio, paintings and even Kleenex Premium Comfort loo paper in his bedroom, but also his own wooden lavatory seat.

Ambushed by the embarrassing anecdote, Camilla, for good measure, added to her husband’s angry riposte by telling bathers on Brisbane’s beach: ‘Don’t you believe that.’

Charles’s discomfort was genuine. He knew his erstwhile host, who had divulged the story to me, was telling the truth.

Similarly, he was left red-faced by another astonished eyewitness’s account in the book of returning to Clarence House with the prince after the opera. ‘Let’s see what’s for dinner,’ said Charles after finishing his martini.

Having disappeared into the dining room, the eyewitness heard Charles’s piercing shriek. Fearing the worst, Camilla had dashed in after him. ‘What’s this?’ trembled her husband, pointing at the food. ‘It’s cling-film, darling,’ Camilla had replied.

Eight years ago, Charles was a hugely controversial character, fuelling doubts about his fitness to be Britain’s monarch.

Not only was he disliked for his treatment of Diana but he was distrusted because his valet, Michael Fawcett, pandered to his extravagant playboy lifestyle.

Not least, Fawcett fed Charles’s voracious demand for cash for his charities, negotiating huge fees from dubious millionaires to sit with the ‘cranky’ prince at his fundraising lunches. That dangerous habit would end with Fawcett’s resignation.

Cash-for-access was among Charles’s greatest sins highlighted in my book.

But the worst was his refusal to listen to the truth. Employees who offered unacceptable advice were fired. And the disillusioned sycophants became the sources of my unprecedented biography about an insecure, spiritualist prince suspected of being incapable of saving the monarchy after his mother’s glorious reign.

Unsurprisingly, Clarence House made no comment about my book. Not only Charles’s staff but the prince himself knew that my detailed description of a self-indulgent meddler publicly battling against Buckingham Palace, government ministers and professional associations was true. The soap opera, the prince knew, had to be terminated.

And terminated it has been, for last week, on his state visit to America, I saw a brilliant king; a confident, charming and wily monarch no longer burdened by the heavy inheritance he has spent most of his life preparing for.

Who but Charles could have aimed calculated digs at his host Donald Trump’s own attack on constitutional ‘checks and balances’ and walk away with the ‘enduring’ relationship between Britain and the US restored?

No other world leader could have simultaneously united the Democrats and Republicans to have 12 standing ovations in Congress and also flatter the truculent President watching from the White House.

It seems incredible that some, including Liberal Democrat leader Ed Davey, argued that the State Visit to celebrate America’s 250th anniversary should have been cancelled.

But regardless of the controversial President, Charles was determined not to allow any feckless Westminster politicians’ dislike of President Trump to sabotage the plan.

After months of deep thought and negotiations, Charles knew that his speeches to Congress and the White House state dinner could define his monarchy. Having made countless orations across the world, he set about drafting his defining thoughts.

Nurtured on his love of BBC radio’s The Goon Show and the ‘By Jove’ witticisms of Bertie Wooster, he crafted a playful and sensitive speech blessed with references to America’s history, Christianity, the monarchy’s endurance and his trust in liberal democracy.

The speech’s partisan gripes, he would have noted privately to the President, were at the insistence of his Prime Minister’s office.

But the King’s real masterstroke was delivered in the shape of a ship’s bell. Presenting the President with the bell of submarine HMS Trump was a pivotal moment in the whirlwind four-day tour, and inconceivable to Whitehall’s desk wallahs.

I appreciate I will not be on the Clarence House Christmas card list for some time yet, but my renewed admiration for Charles is all the more keenly felt because I know how far he has come.

The peevish prince I wrote of – who once described his highly privileged role as ‘utter hell’ – changed his lifestyle. Indulgencies on billionaires’ yachts were ended. His activist battles against vested interests including teachers, doctors and architects disappeared. His crude efforts to manipulate the media were toned down. And the makeover was nearly complete when the Queen died in 2022.

Month by month thereafter, the new King searched for the right tone to establish the Carolean era. Supported by the increasingly popular Camilla, Charles’s speeches and laughing interchanges with the public – so different from his mother’s detached manner – improved his ratings.

And fair-minded Britons rallied to support the stricken monarch after his cancer diagnosis, and praised his bravery once he re-emerged from treatment to star at public events. During that period I sensed that with Camilla’s advice and support he had radically improved. More sober and more secure, Charles was determined to fulfil his destiny as a wise guide to Britain’s future – sorely needed today, more than ever.

Faced with a splintered political establishment and disintegrating traditional values among a multi-cultural population, he knows to tread carefully, avoiding the pitfalls of appearing partisan. The opportunities to voice his opinions and insight in Britain are negligible.

Especially with an unpatriotic Labour Government unable to steer the country away from fractious tribalism, nationalism and the religious fascism that has bred murderous anti-Semitism.

Had Charles been in Britain, he would have been greeted warmly by residents of Golders Green shaken by last week’s attack on two members of the Jewish community there. Certainly not the boos that drummed Keir Starmer’s ears when he visited.

Soon, I hope, he will make a speech addressing his countrymen, urging them that whatever their disagreements, the country must be united in support of its long-standing values of tolerance, liberalism and disdain for extremism.

As he displayed in the US, Charles can bring together those caught either side of a yawning political divide. No longer the Rebel Prince I wrote of, he is now the Serene King.