Charles Spencer, who made it his mission to correct mistakes in her life, said it was “his duty to defend her when I can”. Earl, who said he feared that many people would think her portrayal of the hit show was correct, called last night to allow a fresh investigation of the BBC Panorama interview with his sister to “examine every aspect” of the controversial program. Lord Dyson, a retired Supreme Court judge, has been appointed to lead the investigation into allegations by BBC correspondent Martin Bashir who secured the interview using forged documents to convince Diana that her employees were cheating on her and then covered up. Earl Spencer wrote on Twitter: “As I told the BBC this evening, I am not at all satisfied with the criteria they set for their investigation in the BBC Panorama interview with Diana last night.

“Lord Dyson should be free to study every aspect of the matter, from 1995 to today, as he sees fit.”

The classmate, 56, told three times about his reservations about The Crown in an interview with Alan Titchmarsh to be featured on ITV’s Love Your Weekend tomorrow morning.

He said, “What worries me is that people see a program like this and forget that it is fiction.

Americans tell me they saw The Crown as if they were taught a history lesson. Well, they don’t.

“It’s very difficult. There is a lot of guesswork and a lot of invention, right? You can pin it down on a fact but the parts in between are not real.”

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Of his loyalty to Diana, he said: “I feel it is my duty to defend her when I can. She left me for example as a guardian of her children. And we grew up together – you know that if I grew up with someone it would still be that person, it doesn’t matter what happens to them next.”

Other insiders of the royal family have expressed concern that the series is promoting lies and damaging the reputation of the monarchy.

The show’s creator, Peter Morgan, acknowledged the authorship of the scenes and the dialogue but defended his right to use the artistic license.

“I think all the guys are terrible characterization, except for Emma Corinne, who does Diana well – it’s a good job. Diana as did Claire Foy of The Queen in the first two series,” said former palace press secretary Dickie Arbiter, who described the series. But the rest of the characters are pretty awful.

Peter Morgan admitted that it is neither history nor a documentary. It is a drama.

“And he took absolute liberties with a dramatic license in what he wrote.”

The royal historian and biographer Hugo Vickers complained that the monarchy was wrongly portrayed as an institution that would crush anyone who strayed from the line to survive.

“You better end up hating the royal family,” he said, after watching the fourth series and having so many factual errors. “You can’t help but feel he has an agenda to make the Queen look bad.”