It's believed he will intervene beyond scope of any previous monarchs

It's believed he is turning his attention to overhauling the honours system

He is planning to make a considerable impact in his first six months

There is a growing belief that Prince Charles intends to risk the sovereign's traditional impartiality by making 'heartfelt interventions'

Not long ago, Prince Charles was being driven round the Victoria Monument in front of Buckingham Palace when he started reminiscing to an aide about his childhood. He recalled how, as a boy, he always saw crowds gathering at the Palace to cheer the Queen when she came home from an overseas trip.

‘The people really came out for my mother,’ he said, before adding gloomily: ‘They’ll never come out for me.’

The aide, trying to counter the Prince’s downbeat introspection, pointed out that times had changed, the television age and the internet had seen to that.

But as the car sped on he could see that his point hadn’t got through and that the Prince’s thoughts still weighed heavily on him.

On April 21, the Queen enters her 90th year. On September 9, she is due to overtake Queen Victoria to become Britain’s longest-reigning monarch — by then, she will have been our Queen for 63 years and 217 days.

This is a remarkable personal achievement and the nation will give grateful thanks for a record reign in which she has scarcely put a foot wrong, always making sure her subjects could do no more than guess what she thinks at any time about almost anything. This inscrutability has undoubtedly helped make her so popular.

But what will it mean for Charles?

For him, at 66, this astonishing anniversary is bound to intensify his uncertainty about how the nation will greet him and his style of sovereignty.

Just what that style will be, and whether he will continue to bombard the government of the day with his views, believing the people are behind him in this, is a matter of increasing concern. At this moment, 27 hand-written ‘black spider’ letters which Charles sent to various government departments over a period of eight months are the subject of a legal battle in the Supreme Court over whether their contents should be disclosed.

Dominic Grieve, the former Attorney General, ruled that they should remain secret because the public might conclude Charles had been ‘disagreeing with government policy’. He believes this ‘would be seriously damaging to his role as future monarch because, if he forfeits his position of political neutrality as heir to the throne, he cannot easily recover it when he is King’.

One might expect this forthright view to make Charles back off in alarm. But far from scaling back what some describe as meddling, the growing belief now is that Charles intends to risk the sovereign’s traditional — and to most people crucial — impartiality by continuing to make what are being described by his staff as ‘heartfelt interventions’.

Unfazed by friendly warnings, Charles is said to be planning a strategy to make an considerable impact during his first six months on the throne, whenever that comes.

‘It used to be said that a monarch makes their mark during their first five years, but Charles will be in a hurry because he is no longer young and knows that his reign will have to be short,’ says one former senior aide.

We understand that Prince Charles, a man who is sometimes found sleeping at his study desk in Clarence House after writing and thinking well past midnight, has turned his thoughts to the honours system.

‘This is one area where his opinions might even be welcomed,’ declares one close friend. ‘Why, for example, should people still be invested with an order of a defunct British empire?’ (The CBE, for example, stands for Commander of the most excellent order of the British Empire.)

Charles — ten of his UK charities raised a total of £122.7 million last year — feels that the contribution of philanthropists is not sufficiently recognised with honours.

With shrinking amounts of public money available to good causes, he feels that they are unsung heroes.

It's believed that as King, Charles intends to rename the Royal Victorian Order to the Royal Elizabethan Order as part of a massive shakeup of the honours system

Another of his ideas is to rename the Royal Victorian Order, which is in the gift of the monarch.

He has talked about changing it to the Royal Elizabethan Order. This would be a tribute both to his mother and his grandmother.

Such royal interventions would certainly be welcomed by the many who believe the honours system has become tarnished and devalued in recent times, with gongs going to the wrong people for the wrong reasons.

For his part, Charles thinks that investitures should be more of a celebration. ‘He says it’s a shame that after receiving an honour at the Palace, recipients have to leave,’ says a close figure. ‘How much more appropriate it would be for the occasion to be marked by a party for all recipients.’

To realise such an ambitious and costly plan, however, might mean fewer investiture ceremonies or dishing out fewer honours.

Before he can do any of this, there would be the little matter of his coronation. It is known that he has been planning fundamental changes to the style of his mother’s enthronement in June 1953.

It is many years since he rather prophetically declared that he wanted to be known as ‘Defender of Faiths’ rather than the traditional ‘Defender of the Faith’, meaning the Established Church.

This means he will need to change the Coronation Oath, which requires the new sovereign, as head of the Church of England, to uphold the Protestant religion in this country — a move that will cause a major row even before the crown is lowered on his head.

On September 9, the Queen is due to overtake Queen Victoria to become Britain’s longest-reigning monarch

Few people are unaware by now of the areas where Charles has zealously made a nuisance of himself, though, it must be said, often with great success.

His personal royal-to-royal letter to the Emir of Qatar resulted in Lord (Richard) Rogers’s £3 billion plans for an ultra-modern steel-and-glass re-development of the site of Chelsea Barracks being dumped, and replaced by more traditional architecture — to the relief of most local residents. The Prince opposes genetically modified food production, champions the countryside and urges doctors to embrace alternative medicine, especially homeopathy.

Intriguingly for the first heir to the throne not educated by governesses but sent outside palace walls to school, he has spoken out — dangerously in the opinion of some politicians — in favour of grammar schools.

Says an aide: ‘He likes to say that his mother only cares about dogs and horses while he sees himself as a prince with a heart. He’s jesting, of course, but only partly. He knows that she doesn’t write to government ministers and lobby powerful figures because it’s just not in her nature.’

He, on the other hand, is said to feel a constant need to prove himself. The reason for this is self-doubt, as noted by many who are close to him — though he is not without a certain amount of regal self-importance.

‘Oh, he certainly knows who he is and how people should respond — curtseys and that sort of thing by the ladies,’ says one who knows him well.

‘What worries him is that people don’t take him seriously enough. It’s this that drives him to get involved with difficult issues.

‘His heart’s in the right place, but frankly, he’s looking for praise. I put it down to his childhood.’

Charles’s childhood was one in which, as he told his official biographer Jonathan Dimbleby, he yearned for parental approval. Yet that lingering insecurity is mixed with an unshakeable sense of entitlement.

One fellow guest at a Kensington dinner party found herself on the wrong side of Charles when she headed for the dining room ahead of him and Camilla. ‘There was a princely harrumph, and I instantly realised that I’d made a gaffe,’ she recalls.

So she paused and stood aside as the Prince, wearing monogrammed velvet slippers and murmuring ‘the Duchess goes first’, led the way into the dining room with the former Mrs Parker Bowles.

As she and the other guests were soon to discover, the food he was served was different from everyone else’s — he’d brought his own chef.

That’s something the Queen would never do. But as Charles is prone to say: ‘Just because my mother does it this way doesn’t mean I have to.’

In this, he is referring, of course, to his very different approach to being sovereign. His mother, after all, became Queen when she was 25 while he is still trying to find something to do other than, as he puts it, ‘tread water’ at 66.

Prince Charles and Camilla pictured on their way to a diplomatic reception at Buckingham Palace last year. It is believed a 'quiet constitutional revolution is afoot' as Charles' reign nears

So could the Prince of Wales, who has already once been close to endangering the monarchy through his private life, endanger it again by being a meddlesome King?

‘A quiet constitutional revolution is afoot,’ Dimbleby has said. ‘I predict that he will go well beyond what any previous constitutional monarch has ever essayed.’

A former aide says: ‘He won’t care if people accuse him of interfering, but he does know he will have to be more cautious.

‘His view is that he was sent by God to change the world, and he sees it as his royal duty to push his views.’

There is certainly a spiritual dimension to Charles’s crusading. In his 2010 book Harmony: A New Way Of Looking At Our World, he rails against a ‘frenzy of change’ that gripped the industrialised countries in the Sixties.

‘Even as a teenager I felt deeply disturbed by what seemed to have become a dangerously short-sighted approach,’ he writes. He says that by the Seventies (when he was in his 20s and in the Royal Navy): ‘I could see very clearly that we were growing numb to the sacred presence that traditional societies feel very deeply.’

Few would have suspected that Charles was thinking with such profundity in those years when he was known as ‘Action Man’ and squiring a string of pretty, well-connected girls.

Prince Charles pictured decorating officer Patrick Hyde with the Military Cross in July last year

It was during this period that he first met Camilla Shand, the young woman who would go on to cast such a long shadow over his marriage to Diana, Princess of Wales.

Until now, friends of the Prince were convinced he was involving himself with such fervour in public issues because he knew that once he became King such activity would have to stop. But now something has changed.

Perhaps it is that the old conventions are dying. Or perhaps — as friends suggest — he has observed his mother and decided that with her being such a model monarch, so many opportunities for improvements in certain areas of British life have been lost.

‘He sees the tradition of discretion embodied by the Queen as relatively recent, and feels that traditions can change,’ says a long-standing member of his household. So he has let it be known that as King he will be significantly more than just a figurehead.

The problem, courtiers worry, is that the phrase ‘heartfelt interventions’ can be so dangerous. This may not matter with issues such as the Prince’s war on ugly modern architecture or organic food, but he has developed a sharp interest in highly political areas as well.

Since the beginning of 2012, he has held some three dozen meetings with Government ministers. The subjects discussed are confidential, but he has previously lobbied ministers on such incendiary issues as fox-hunting, the NHS, grammar schools, farming policy and human-rights laws.

And, alarmingly, he has even made it known privately that he favours a proportional representation system of voting at elections — a particularly controversial stance.

In a letter to Lord Irvine, then Lord Chancellor in the Labour government, which was leaked to the Mail, he rubbished the Human Rights Act, suggesting it was ‘only about the rights of individuals (I am unable to find a list of social responsibilities attached to it) and this betrays a fundamental distortion in social and legal thinking’.

In another letter to Lord Irvine, he expressed his concern that the Act ‘will only encourage people to take up causes which will make the pursuit of a sane, civilised and ordered existence ever more difficult.’

He added: ‘I, and countless others, dread the very real and growing prospect of an American-style personal injury culture becoming ever more prevalent in this country.’

One former member of the Prince’s private office says: ‘I used to think the Prince had the right approach getting involved in things and that the Queen’s neutral and ultra-cautious style was wrong.

‘But I’ve changed my mind because the political landscape is completely different. So many parties are now jostling for a share of power that hung parliaments are likely to become the norm, and it could even fall to Charles, as King, to choose a prime minister.

‘I worry that people will say: “We don’t want him making the choice because we know what he thinks.” No one knows what the Queen thinks about anything, and Charles will need some of his mother’s conservatism if he’s to avoid a constitutional crisis.’

But no one underestimates Charles’s stubborness. ‘If he believes in something and grabs hold of it, he won’t let go,’ one figure observes. ‘And he is very aware of his influence.’

It is an influence which he has been accused of misusing. There was his clash with Professor Edzard Ernst over a 2005 report commissioned by Charles which concluded that alternative medicine — which he has championed for decades — should be given a greater role in the NHS.

Since the beginning of 2012, Prince Charles has held some three dozen meetings with Government ministers. Here he is pictured with Camilla when the pair's car was set upon by student protesters

Professor Ernst, the chair of complementary medicine at Exeter University since 1993, described the report as ‘completely misleading rubbish’.

This brought a letter of complaint to the university from Prince Charles’s then private secretary, Sir Michael Peat. As a result, the distinguished academic was accused by the university of ‘breach of confidence’ for disclosing the report before it had been published.

‘I was interrogated, investigated and treated like dirt for 13 months, and exonerated in the end,’ he writes in his book A Scientist In Wonderland, published earlier this month. But he had had enough. In 2011, he took early retirement and his department was closed.

‘Prince Charles’s attempt to silence me had been successful,’ Professor Ernst now says. ‘As a King, he is determined to make his mark, but he seems to have no idea of his own intellectual limitations.

‘My feeling is that he cannot wait to be in a position where his bizarre views must be taken a bit more seriously. His history of trying to exert an influence on issues he holds dear is too long and strong simply to disappear when he ascends the throne.’

He’s certainly right about it being too strong. As he approaches his 67th birthday in November, Charles, like the Chinese, is known to place increasing value on the wisdom of old age. This at a time when our most senior politicians such as David Cameron and Ed Miliband are in their 40s.

‘He feels his age gives him the wisdom and the right to express his views to politicians who are so much younger than he is,’ says a palace courtier.

Much of this has its roots in the lasting influence of his one-time guru, the late Sir Laurens van der Post, who impressed on him that every passing decade made him not ten years older, but ‘ten years wiser’.

What worries even fans of the Prince’s interventions is whether his judgment is fine-tuned enough to overcome his passion.

He has made political gestures as Prince of Wales that would be quite impossible as King. He pointedly snubbed a Buckingham Palace state banquet for the visiting president of China in 1999 because of that country’s attitude towards Tibet and its offhand treatment of the Dalai Lama, whom he reveres.

One area where ministers fear there is potential for a clash is what Charles (pictured with Camilla) sees as the greed of supermarkets virtually blackmailing dairy farmers to sell their milk incredibly cheaply

Since then, many of Charles’s views have hardened, but he could hardly do that again when he is head of state.

One area where ministers fear there is potential for a clash is what Charles sees as the greed of supermarkets virtually blackmailing dairy farmers to sell their milk so cheaply that many of them can barely make a living.

He certainly did not restrict these views to ‘black spider’ letters written privately to government ministers. He put them in a signed article for Country Life, angrily declaring: ‘It cannot be right that a typical hill farmer earns just £12,600 . . . while the big retailers and their shareholders do so much better out of the deal, having taking none of the risk.’ Hill farmers, he wrote, are ‘penalised for choosing such a vital profession’.

No one seriously imagines that as King Charles III he will step back from these views. Similarly, he is vehemently opposed to wind turbines blighting the countryside, and refuses to allow them to be built on his Duchy of Cornwall Estates.

There is a bitter irony here. As King, his income would be a share in the profits of the Crown Estates — under a new system for which he was agitating over more than 20 years.

And, significantly, offshore wind farms have to pay rent to the Crown Estates, and there are likely to be 7,000 of the giant turbines round the UK coastline by the year 2020. This could make him the richest monarch in British history.

‘People must judge the Prince on what he has done on behalf of the people of this country,’ declares one of his senior former aides.

‘Yes, he has made mistakes in his private life and I think he now acknowledges this.

‘Is he misguided on occasion? Possibly. But he believes he is trying to make things better. Even as King he would want people to know that, and, ultimately, to have that as his epitaph.’

Haunted by guilt, a Prince of Wails

The new book by Catherine Mayer called 'Charles, the Heart of a King' (pictured), describes him as one of the world's 'least understood figures'

A picture of one of the world’s ‘least understood figures’ emerges in a new biography of Prince Charles.

It quotes actress Emma Thompson, a close friend, as saying: ‘We talk a lot about the guilt of privilege. Sometimes I think he’s driven by guilt.’

It describes Charles as a prince who will ‘never be remote and silent like his mother’ and is driven to improve the human condition. He’s quoted as saying: ‘I want to raise aspirations and recreate hope from hopelessness and health from deprivation.’

Author Catherine Mayer talks of a prince who ‘rarely recognises his own achievements’ and is described by one of his inner circle as ‘a glass-half-empty man’, a ‘Prince of Wails’.

Another close figure talks of Charles’s temper. ‘The royal rage, I call it. Here comes the royal rage.’

Ms Mayer discovered a man whose despondency, she said, ‘has been profound’ at the darkest moments in his life.

Prince Charles confided to her: ‘Each thing I did, you had to meet another lot of people who have all sorts of views of you beforehand, all sorts of prejudices.’

The private Charles, however, may have a luxury lifestyle but, says former private secretary Clive Alderton: ‘When you are having tea with him, (he) gets any leftovers wrapped up and brought back for his next meal, and the next one. I’ve rarely met someone who is so frugal, not in the sense of meanness but an absolute allergy to waste, and in particular waste of food.’

Nor is this frugality confined to his eating habits.

In the biography, Ms Mayer (editor at large for Time magazine) reveals how, one hot summer, he syphoned dirty water through a pipe from his bath at St James’s Palace onto his garden below.

Of course, there is, as ever, a fundamental contradiction in such behaviour.

The book says: ‘He lives high on the hog . . . but combines the showiness of royal life — the banquets, the acreage of cut flowers, retinues larger than most other family members employ — with a frugality absorbed from his parents and from the austere schooling at Gordonstoun, and more consciously informed by his environmental concerns.’

Ms Mayer describes him wanting to be King as ‘the least of his ambitions’ but, confusingly, goes on to say he is ‘gnawingly impatient’ to accede to the throne.

Yet he is said to ‘dread’ the death of his parents, and still mourns ‘with a startlingly raw grief’ his grandmother (the Queen Mother), his great-uncle Earl Mountbatten (murdered by the IRA) and his guru Sir Laurens van der Post.

On a lighter note, some of Charles’s staff refer to wealthy dinner guests who the Prince entertains in return for their donations to his worthy causes as ‘Bond villains’. And a member of his household at Clarence House reveals there is a competition there ‘to see how deep you can curtsey without falling over’.

Ben Elliot, Camilla’s nephew, describes Charles as warm and ‘extraordinarily tactile’. The Prince, we learn, made the tea at the family gathering after the funeral of Camilla’s adventurer brother Mark Shand last May.

Charles is revealed also to organise surprise parties for friends and people he admires — one of these being a joint 80th birthday party at Clarence House for the playwright Sir Ronald Harwood and the actress Dame Maggie Smith.

Extracts of the book, Charles: The Heart Of A King, have been released on Kindle in advance of publication next Thursday.

The publisher boasts that Ms Mayer’s work is ‘based on exclusive interviews with members of the Prince’s inner circle and underpinned by rare direct access to the Prince himself’. It is believed she has interviewed the Prince and more than 50 of his friends, family confidantes and staff.

Ms Mayer says her access to him included trailing him for six months, ‘struggling to keep up as he strode across muddy fields’, and she dined with him at Dumfries House in Scotland. The Prince’s cousin Tim Knatchbull (whose twin brother, Nicholas, was killed aged 14 by the same bomb that blew up Lord Mountbatten) describes Charles as ‘an expert mimic . . . a sort of Rory Bremner . . . ’

Although ‘fundamentally serious’, Charles is seen as a natural comic. One of his favourite gags involves instructions given to air passengers afraid of being marooned on a desert island.

Prince Charles and Prince Harry share a laugh at the start of the Invictus Games last year. Charles is said to be a natural comic and to enjoy a laugh

The advice begins with them being told always to make sure they’ve got a cocktail shaker and the means of making a martini. The punch-line centres on the moment they start to mix the drink — ‘I can guarantee within a minute someone will jump out of a tree and say: “That’s not the proper way to mix a martini!” ’

His delight in the absurd was witnessed at Prince Harry’s 13th birthday party, when he was ‘laughing uproariously while competing with Emma Thompson in a game in which players donned hats covered in Velcro to try to catch felt-covered balls’.

Incidentally, Ms Thompson is also quoted as saying that dancing with Charles was ‘better than sex’.

If Ms Mayer has the right birthday for Harry, and the party took place on the day itself, it would have been just 16 days after Princess Diana’s death.

In what she calls her ‘fundamental reappraisal’ of Charles and Diana’s marriage, Ms Mayer concludes that in the so-called War of the Waleses, the Prince ‘could never match Diana’s lustre or her blood-lust’.