As the Prince of Wales, Charles admitted he was an “inveterate interferer and meddler” and his outspoken opinions have been fully and freely aired over the years.
GM crops, nanotechnology, monstrous carbuncles, the environment, farming and complementary medicine have all provoked comment during his lengthy stint as Britain’s longest-serving heir apparent.
But as he turned 70, Charles insisted his meddling would not continue when he acceded to the throne.
“No, it won’t. I’m not that stupid,” he told a BBC documentary in 2018. “I do realise that it is a separate exercise being sovereign. So, of course I understand entirely how that should operate.”
Concerns were often raised as to how his opinions would feature when he became monarch, and he will be under intense scrutiny as such as the new King.
Sources close to Charles told The Guardian in 2014 that he would break with tradition and make “heartfelt interventions” in national life.
They said he would not follow his mother’s discretion on public affairs, but instead speak his mind on issues such as the environment.
Catherine Mayer’s 2015 unauthorised biography of Charles said the prince was planning to introduce a “potential new model of kingship” but that the Queen was concerned about the potential style of the monarchy under her son.
But Charles’s senior aide at the time, principal private secretary William Nye, came to his defence, saying Charles understood “the necessary and proper limitations” on the role of a constitutional monarch.
As head of state, Charles is a non-political figurehead and must remain strictly neutral.
Royal advisers may have attempted to curb his controversial views in the past, but the prince often hit the headlines, particularly over his lobbying of ministers.
Politicians were said to have regularly moaned about the number of letters they received from the crusading prince.
The notes were known to recipients as the “black spider” memos on account of Charles’s handwriting, with the prince enthusiastically detailing his beliefs on particular political topics, using lots of underlining and exclamation marks.
In 2002, he found himself at the centre of a constitutional row following revelations he had been “bombarding” ministers with letters attacking government policy.
One minister said that the prince had become so involved in politics that he wrote a letter a week to the government.
It emerged he had written a series of letters to the then lord chancellor, Lord Irvine of Lairg, expressing concern over the growth of the “compensation culture” and the Human Rights Act.
The prince warned that human rights legislation is “only about the rights of individuals …and this betrays a fundamental distortion in social and legal thinking”.
Charles also took issue with the “degree to which our lives are becoming ruled by a truly absurd degree of politically correct interference”.
He has not always wanted his views to be made public.
Letters he wrote to a number of government departments between 2004 and 2005 became the subject of a protracted legal battle over whether their contents should be disclosed.
Twenty-seven letters, 10 from Charles to ministers, 14 by ministers and three letters between private secretaries, were released in 2015 following a 10-year campaign by Guardian journalist Rob Evans to see the documents after a freedom of information request.
The publication of the correspondence showed the prince lobbied then-prime minister Tony Blair and other ministers on a range of issues from badgers and TB to herbal medicine, education and illegal fishing.
He also tackled Mr Blair over a lack of resources for the armed forces fighting in Iraq.
There were no handwritten “black spider” letters among the batch released.
A further six letters were unveiled later in 2015, showing Charles wrote to ministers between 2007 and 2009 about topics such as hospital food, affordable rural housing and climate change.
It was also out and about on engagements that the prince caused controversy.
In 2014, he sparked a diplomatic storm after comparing Russian President Vladimir Putin to Hitler during a visit to Canada.
Charles was speaking to Canadian museum volunteer Marianne Ferguson, who told him how her Jewish family fled the Nazi occupation of Danzig at the outset of the Second World War, when he drew a parallel with Russia’s annexation of the Ukrainian peninsula of Crimea.
Russia responded angrily to the comparison, leaving the Foreign Office attempting to head off a diplomatic row by holding talks with Russia’s furious deputy ambassador.
Even Mr Putin, in a direct message to Charles, publicly branded the comments “unacceptable” and said such remarks were “not what monarchs do”, leaving royal relations with Russia in tatters.
In 2006, Charles’s former aide Mark Bolland revealed at the High Court that the prince saw himself as a “dissident” working against current political opinion.
Charles’s journals, around which the case centred, published in the Mail on Sunday revealed that he made unflattering comments on the handover of Hong Kong to China when he described Chinese diplomats as “appalling old waxworks”.
It was his friendship with the exiled Tibetan leader the Dalai Lama which led him to miss out the return banquet during the Chinese president’s trip to the UK in 1999.
A Clarence House spokesman insisted years later that the prince’s actions in 1999 were not a snub.
But as Mr Bolland revealed in court: “I was given a direct and personal instruction by the prince to draw to the media’s attention his boycotting of the banquet.”
Such snubs are simply not an option now that he is King.
He must keep his feelings about prime ministers, presidents and countries across the world to himself and fulfil his duty as sovereign.
In 2007, when Channel 4’s Dispatches programme Charles: The Meddling Prince raised questions about his suitability for the throne, his then senior aide Sir Michael Peat dismissed the claims that Charles abused his position by secret lobbying.
“It would be more damaging to the monarchy if he did not use his position to help with issues,” he said.
Sir Michael added: “It hardly needs saying that the Prince of Wales, of all people, knows that the role and duties of the heir to the throne are different to those of the sovereign and that his role and the way he contributes to national life will change when he becomes king.”
One contributor to the programme, Lord Wedderburn, warned of difficulties.
“If in fact nothing changed and he became king, then there would be a most almighty fuss and controversy, and eventually the whole fabric of the constitutional monarchy could be threatened,” he said.
There was outrage in 2004 when the prince blamed the education system for making people “think they can all be pop stars”.
He wrote in a memo disclosed at an employment tribunal: “What is wrong with everyone nowadays?
“Why do they all seem to think they are qualified to do things far beyond their technical capabilities?”
During the foot-and-mouth disease crisis, the prince attempted to prevent the cull of cattle.
The then-National Farmers Union leader Sir Ben Gill revealed that it had been Charles who was behind the biggest push for vaccination.
Clarence House said in a statement at the time: “Part of the Royal Family’s role is to draw attention to issues on behalf of us all.”
In 2004, the prince was lambasted for declaring that continuing research into nanotechnology could result in a thalidomide-style disaster.
Charles wrote a piece in a Sunday newspaper which drew a link between nanoscience, the groundbreaking study of matter a millionth of a millimetre wide, and a medical catastrophe which led to thousands of children being born with deformities.
Nanoscientists declared he had a “primal fear of technology”.
“Why are we listening to Prince Charles? What are his scientific qualifications?” one remarked.
There were earlier suggestions that Charles feared out-of-control nanobots reducing all the Earth’s biomass to “grey goo”.
But he later denied ever using the expression, insisting: “I do not believe that self-replicating robots, smaller than viruses, will one day multiply uncontrollably and devour our planet.”
Charles’s concerns over GM foods are well known.
Mr Bolland revealed that his “vociferous campaign” against them was a prime example of the way the prince readily embraced politically contentious issues.
In 2002, Charles said in a speech: “I think it’s going to cause the most appalling problems … we’re tampering with something fundamental, trying to redesign nature.”
It contrasted at the time with an address by the then prime minister Tony Blair to scientists in which Mr Blair said he could find no evidence “of serious health risks” in GM crops.
In 2002, a leaked letter to Mr Blair ahead of a large pro-hunt march in London showed that the prince had relayed a Cumbrian farmer’s views that the campaigners were being treated worse than other minorities.
“If we, as a group, were black or gay we would not be victimised or picked on,” was the reported comment.
The prince was also alleged to have said that he might as well leave Britain if fox hunting was banned.
His letter writing led to him being dubbed “Angry of Windsor” and the “Prince of Wails” by the newspapers.
Charles has also been outspoken on architecture.
Speaking in 1984 about the proposed National Gallery extension, he stated: “What is proposed is like a monstrous carbuncle on the face of a much-loved and elegant friend.”
One architect later said it had taken the industry the best part of 20 years to recover from the effect of that speech.
In May 2009, he became embroiled in a new row concerning a development at Chelsea Barracks.
The prince contacted representatives of the Qatari royal family, who own the London site, suggesting that Richard Rogers’ designs for a £1 billion housing scheme on the site of the former barracks were “unsympathetic” and “unsuitable”.
He was criticised by architects for threatening the “democratic process” and using his “privileged position” to have his say on the design.
In the end, Charles won the battle when the developers withdrew their planning application.
Former planning minister Nick Raynsford accused the prince of setting a “very dangerous precedent” and behaving in an “almost feudal” way by intervening in the project.
In 2005, the prince warned it was wrong to knock down old buildings where they could be restored more economically.
It came amid plans by John Prescott, the then deputy prime minister, to demolish up to 400,000 homes in the Midlands and the North.
Although Clarence House insisted his remarks were not aimed at government policy, they were interpreted as registering disapproval of Mr Prescott’s proposals.
In 2008, the prince declared frankly, referring to his involvement in a restoration project in Bradford: “Being an inveterate interferer and meddler I couldn’t possibly stand back and do nothing.”
In 2014, he spoke out about the winter flooding on the Somerset Levels, calling it a “tragedy” that “nothing happened for so long”.
His comment came amid growing anger over a perceived lack of government action to help flood-hit areas.
He also described the flooding as a “classic example” of what happens if society pays “little attention to the accumulating impact of climate change”.
Charles has been a passionate supporter of everything from organic food and farming to mutton, albatrosses, healthy hospital design and homoeopathic medicine.
He has sought to make a difference during his decades as heir to the throne, campaigning for the environment, setting up The Prince’s Trust and promoting multi-faith tolerance – something he could not have done had he been a young king.
In 2018, he said: “You know, I’ve tried to make sure whatever I’ve done has been non-party political, and I think it’s vital to remember there’s only room for one sovereign at a time, not two. So, you can’t be the same as the sovereign if you’re the Prince of Wales or the heir.”
His views are steadfast and have been a major part of his life and focus.
The new King will meet the Prime Minister every week at a private audience where he may express whatever opinions he likes.
In fact the British monarchy website declares it is the sovereign’s “right and a duty to express (his) views on Government matters”.
But it adds that “having expressed (his) views, The (King) abides by the advice of (his) ministers”.