One will not be silenced: Charles rides into battle to fight a new campaign

The Prince of Wales warns about the use of nanotechnology today, knowing the backlash may be harsh. Geoffrey Lean reveals how Charles came to believe he must speak out, despite risking derision

It was the only time I have had kidneys for breakfast. It was certainly the only time I have eaten them in the company of the Prince of Wales. Our meeting took place on the morning of a general election, so I asked the Prince how he would have cast his vote, if he had one. Inevitably, he sidestepped the question - and the conversation moved quickly to the launching of his latest campaign.

I forget his exact words, even the cause. But I vividly remember the revelation - as he mentioned how he had to get his courage up before each one - of how much the ensuing controversy would cost him, personally. And the realisation that far from shooting his mouth off - as is the popular perception - he spends months preparing his interventions.

A sensitive man, he does not relish controversy and is hurt by suggestions he is an ignoramus with no business raising his chosen issues. One adviser says: "It costs him when interventions he makes are not accepted as being in good faith."

But he does not read newspapers and has come to accept the jibes as "inevitable". He likes to recall how a proposal he made for improving fishing practices resulted in him "being accused of wanting to ban fish and chips".

Prince Charles feels justified by his record. He first raised concerns about genetic modification in 1995, for example, pinpointing precisely the issue - the effects on wildlife in GM fields - that has seen off the technology in this country.

Last week the Prince was attacked for suggesting alternative therapies might help fight cancer. Public attention was also focused on the Queen's tribute to the Princess of Wales at the opening of her memorial fountain, where Charles sat looking thoughtful. But away from the cameras he was putting the final touches to his latest sally, which we publish exclusively today (see page 25).

Few people may have heard of nanotechnology but this is an issue that will affect the future more than the subjects of any of his previous campaigns.

"He wants to ensure that there is at least some public debate before the widespread adoption of a technology whose consequences are as yet unknown," says his close adviser, Jonathon Porritt. A former head of Friends of the Earth, Mr Porritt also advises the Prime Minister on the environment. Charles's engagement with nanotechnology began early last year, as Zac Goldsmith - the multimillionaire son of Sir James and editor of The Ecologist - was preparing a special issue of the radical magazine founded by his uncle, Teddy Goldsmith, more than 30 years ago. At 29, Zac combines the energy and idealism of youth with the clout that comes with a £100m inheritance.

He came across a new report by the Canada-based campaigning Etc Group. Mr Goldsmith says he "sent 40 or 50 copies of it to people who I thought might be interested, including the Prince of Wales". A request quickly came back from the Prince's office for seven more copies to be distributed to advisers, and Charles began to focus intensively on the issue.

Advisers such as Mr Porritt warned, however, that this was a much bigger subject than GM. The Prince decided to convene a group of experts from both sides of the debate, as he often has. But in 2003 the preparations went off "half cock", as one top official put it, when news of the planned meeting came out. The coverage concentrated on perhaps the most spectacular - but much the most improbable - scare about the technology: that it could give rise to billions of self-replicating robots that could rapidly reduce the entire world to "grey goo".

"I have never used that expression," says Prince Charles, but, within days of the story breaking, newspapers were unanimous in wrongly reporting that this was his main concern. Dr Ian Gibson, chairman of the House of Commons science committee, told him to "keep his nose out". Commentators called him the "Jeremiah of grey goo" and opined that it really existed only "between the Prince's ears".

Yet the row stimulated the Government and the Royal Society - Britain's premier scientific institution - to set up a committee, including Mr Porritt, to investigate the technology. Meanwhile the Prince persuaded the Royal Society to assemble a group of senior scientists and got Mr Goldsmith - not previously centrally involved in his campaigns - to convene some sceptics. They met on 17 March for dinner at Clarence House.

Kensington Palace refuses to disclose who was there or what was discussed, on the grounds it was "a private dinner". But the scientists around the table were led by Lord May, president of the Royal Society and an old sparring-partner of the Prince's over GM, and included Professors Ann Dowling of Cambridge and John Ryan and George Smith of Oxford.

The sceptics included Mr Porritt and Mr Goldsmith, who brought Pat Mooney and Jim Thomas of the Etc Group, and Dr Vyvian Howard of Liverpool University. No one was invited from industry, though it is largely driving developments in nanotechnology. The discussion was frank, but not hostile. The evening ended with broad agreement, as one put it, that there were "major upsides and downsides to the technology" and that it would need to be regulated.

As a result the Prince decided to take up the issue, and wrote today's article. He knows what the reaction is likely to be. When he spoke up about GM, Professor Steve Jones of University College London, called him "a classic woolly thinker" who should "go back to school and do more A-levels". Yet the scientific evidence has supported the Prince.

During the long wait to be king, Charles has reinvented the role of Prince of Wales to become the first campaigner-prince, driven both by a sense of duty to the country and his own interests in spirituality, architecture and the environment. His right to speak out has been questioned by republicans and those who wonder what he is doing interfering in their field of expertise. But as a pioneer organic farmer, the Prince now looks on with satisfaction at the booming demand for chemical-free food. And as an early proponent (with the IoS) of vaccination in the foot-and-mouth crisis, he can feel justified as ministers now write the ideas they dismissed into official policy.

Today he opens up a new front. It may prove to be the most difficult and controversial one of all.