The Prince of Wales was angry after his attempt to persuade Tony Blair's government to expand grammar schools fell on deaf ears, former education secretary David Blunkett has said.
Charles "didn't like" it when his request was refused, according to a BBC Radio 4 documentar, The Royal Activist,
"I would explain that our policy was not to expand grammar schools, and he didn't like that," Blunkett said.
"He was very keen that we should go back to a different era where youngsters had what he would have seen as the opportunity to escape from their background, whereas I wanted to change their background."
Blunkett added: "I can see constitutionally that there's an argument that the heir to the throne should not get involved in controversy; the honest truth is I didn't mind.
"If you are waiting to be the king of the United Kingdom, and you've waited a very long time, you genuinely have to engage with something or you'd go spare."
Anti-monarchy pressure group Republic said Charles had a "political agenda" which was "at odds with a lot of voters" and the Government should come clean about the extent of royal influence.
Chief executive Graham Smith said: "Charles and his aides have had numerous meetings with government ministers since the last election. We know this lobbying is carrying on. What the public don't know is how much policy is being shaped by Charles.
"He is wading into some very controversial issues such as grammar schools, GM crops and alternative medicine. The public has a right to know whether his agenda is having an influence on government decisions.
"The public needs to know the details; what is Charles lobbying for and is he getting his way? Whatever ministers might say to the BBC this royal interference is not acceptable.
"Of course Charles has a right to his opinion, but he can't have it both ways. Either he's a politician or a prince. If he wants to be involved in politics he needs to be accountable to the people.
"Charles is engaging in an orchestrated and concerted effort to influence government behind closed doors and beyond public scrutiny. He wants the power but doesn't want the public to know what he's up to.
"The Government needs to come clean, reveal the extent of royal influence and the nature of Charles's lobbying of ministers. Without that transparency our democracy is under threat."
I never understand why ministers don't just treat Prince Charles like the pampered dillettante he is: http://t.co/IcXPzP8fkQ— Kevin Meagher (@KevinPMeagher) June 29, 2014
Delighted to see Prince Charles getting involved in policy making and looking forward to seeing him stand for a seat in parliament next year— Mark Thompson (@MarkReckons) June 29, 2014
Days after hacking victim David Blunkett tells of pain of having privacy invaded he makes public private chat with Prince Charles #hypocrisy— Nigel Pauley (@nigelpauley) June 29, 2014
Former environment minister Michael Meacher said he and the Prince "would consort together quietly" to affect policy on climate change and genetically modified crops.
"I knew that he largely agreed with me and he knew that I largely agreed with him," said Meacher. "We were together in trying to persuade Tony Blair to change course."
Another former Labour cabinet minister, Peter Hain, said they shared an interest in complementary medicine.m"He had been constantly frustrated at his inability to persuade any health ministers anywhere that that was a good idea, and so he, as he once described it to me, found me unique from this point of view, in being somebody that actually agreed with him on this, and might want to deliver it."
Hain added: "When I was Secretary of State for Northern Ireland in 2005-7, he was delighted when I told him that since I was running the place I could more or less do what I wanted to do. "I was able to introduce a trial for complementary medicine on the NHS, and it had spectacularly good results, that people's well-being and health was vastly improved.
"And when he learnt about this he was really enthusiastic and tried to persuade the Welsh government to do the same thing and the government in Whitehall to do the same thing for England, but not successfully."
The Prince's policy interventions were supported by former prime minister Sir John Major, who said: "I think it is encouraging that the Prince of Wales is entirely free from his unique perspective to write to ministers or the prime minister in a way that is invariably intended to be helpful, and I think to cut that off, or to make sure those letters are much more bland than they otherwise might be, would be a loss."
Sir John also revealed that he occasionally changed policy as a result of discussions with the Queen - although he would not be drawn on the specific times this took place.
Asked if he remembered being influenced by the Queen, Sir John said: "I think every prime minister can think that, and can think of occasions where that happened...
"But the answer is yes of course. It would be very foolish indeed not to be influenced."