Prince Charles agonised over whether to call off his wedding to Diana, the Daily Mail can reveal today.
Despite realising that he and the aristocrat's daughter were incompatible, he felt powerless to do anything about it.
Breaking off the engagement "would have been cataclysmic", he said.
And he described himself as being "permanently between the devil and deep blue sea".
Years later and still troubled by the tragedy of the marriage, he wept tears of frustration as he told friends: "I desperately wanted to get out of the wedding in 1981, when during the engagement I discovered just how awful the prospects were."
The story of the heir to the throne's anguish over the doomed marriage is told in an explosive new royal biography to mark his landmark 70th birthday next month, the Daily Mail reports.
It reveals how Charles wants to challenge "pernicious lies" about him – which he believes were orchestrated by Diana herself.
Veteran royal correspondent Robert Jobson, the book's author, has accompanied the prince for 18 months on tours around the world.
Jobson, who has reported on the royals for almost three decades, has spoken to many current and former members of his staff – and to Charles himself.
The result is a compelling book full of ground-breaking new material which is being serialised in the Mail today and next week.
Among the sensational revelations are:
How Charles is already 'Shadow King' and the Queen could stand aside at 95, allowing him to become Regent.
• How shocked Charles was by his sons' level of belligerence;
• His dismay at being excluded from the boys' 2017 documentary about Princess Diana;
• Harry's meltdown ahead of his wedding to Meghan Markle;
• How Charles "talks to the dead", including Earl Mountbatten, his mentor murdered by the IRA;
• His disdain for Tony Blair, whom he once mockingly referred to as "our magnificent leader;"
• His despair at the Iraq War, telling friends Blair had behaved like George Bush's poodle;
• How he disagrees with the burka bans introduced by France and other European countries;
• His distrust of US evangelicalism and the lobbying power of giant American corporations;
• His fear that America's chemically-gown corn was fuelling that country's ever-growing obesity and diabetes crisis;
Charles emerges from this powerful portrait as a contented figure, who is far from bitter but has strong views on some of the most vital issues facing us today.
But it is the revelation about his heartache over Diana that is bound to raise eyebrows.
Ever since Diana confided in biographer Andrew Morton we have known that the princess wanted to back out of the marriage.
But until now we had no idea that the prince did too.
It is at times brutal reading as the book details Charles's sadness at his failure to confront what was to become an unfolding tragedy.
Jobson says that the prince knew within just a handful of meetings they were ill-suited.
He told friends how in the weeks leading up to the wedding when he tried to explain his daily routine, Diana seemed incapable of grasping what he was saying.
The more he saw her, the more he realised he was making a mistake.
Charles said he blamed no one – neither his father nor his mother – for his inability to back out of the engagement.
Instead he blamed his own failure to get to know Diana properly beforehand.
The princess said they had met no more than 12 times before their engagement in February 1981.
According to Jobson, the prince is haunted by sadness over the doomed marriage to this day.
But in one regard the sorrow is matched by his fury at lies peddled about him.
He blamed Diana for this, telling a friend of "unbelievable and pernicious lies aided and abetted by somebody rather close to me [who] lived hand to mouth with the Press".
The most hurtful lie that has echoed down the years was the claim that he had secreted the then unmarried Diana on board the Royal Train for a tryst.
The story was untrue as was another – that Camilla was smuggled into Buckingham Palace the night before his wedding to Diana.
Other stories he labels "lies and inventions" include claims that Camilla had been pelted by bread rolls by shoppers in a store in Chippenham, Wiltshire.
This, he describes as a "persistent lie".
Last night fellow biographer Penny Junor, who wrote a well received book on Camilla last year, said: "This is the first time I have heard of [Charles] openly talking his doubts about marrying Diana while they were engaged. I certainly didn't know he had mentioned any misgivings to friends."
Ingrid Seward, editor in chief of Majesty magazine, said: "This effectively rewrites the Charles and Diana story with its significant interpretation that Charles as well as Diana felt the marriage was doomed from the beginning."