Princess Diana told friends that she “hated Prince Philip ” and warned her sons to not be like him, an upcoming book has claimed.

Royal biographer Ingrid Seward said Diana and the Duke of Edinburgh exchanged a series of letters about Prince Charles’ affair.

She details their exchanges in upcoming biography Prince Philip Revealed, being serialised in the Mail on Sunday.

In one letter, Ingrid said Philip “asked her to reflect on why her husband had returned to his old flame.”

He is believed to have been referring to Charles ’ now-wife, Camilla, Duchess of Cornwall, who rekindled her relationship with her former boyfriend after his marriage had "irretrievably broken down" in 1986.

Eventually, the letters became “too much” for Princess Diana, Ingrid claimed.

Royal biographer Ingrid Seward claims Princess Diana told her she 'hated Philip'

She wrote: "Although the precise details of all the letters are not available, the substance of Philip’s missives was that Charles was wrong to have returned to Camilla, but that Diana, too, was wrong to have other lovers.

"As the letters went to and fro, he asked her to reflect on why her husband had returned to his old flame.

"Eventually it was too much for Diana. Unable to take criticism of any sort, she decided she hated Philip (that is what she told me) and his mission failed."

She claims Princess Diana also told her about a row Sarah Ferguson is alleged to have had at dinner with Prince Charles

At the time, Philip denied he had sent harsh letters to Diana and former butler Paul Burrell later commented that Philip "did more to save the marriage" than Prince Charles.

Diana tragically passed away on August 31, 1997 , after the limo she was in with boyfriend Dodi Fayed crashed in a tunnel in Paris.

Dodi was killed immediately, but Diana was pronounced dead in hospital.

She was buried in a ceremony watched by billions of people 6 days later on September 6, 1997.

Ingrid has said Philip 'asked her to reflect on why her husband had returned to his old flame'

After her death, a former friend of Diana's, the late American billionaire Teddy Forstmann, came forward and said Di told him she "hated" Philip.

He said: "She hated Prince Philip."

Ingrid writes: "When I saw the Princess at Kensington Palace shortly before her death, she told me the same, adding that she had warned her sons: .Never, never shout at anyone the way Prince Philip does'.

Prince Philip 'doesn't look back', he has said

Ingrid claims Princess Diana also told her about a row Sarah Ferguson is alleged to have had at dinner with Prince Charles.

She writes: "Philip became more and more irritated with Fergie and what he described as her ‘antics’. He began to take every opportunity to criticise her, and disliked what he considered her informality with the staff.

"She became nervous in his presence, and when she tried to make polite conversation, he let his contempt be known. He also goaded Andrew, picking on him at every opportunity.

"Fergie found this so insulting, she told me, that it gave her the courage to admonish Philip when he rebuked Andrew over dinner one day.

"She told him in no uncertain terms that he couldn’t speak to Andrew like that."

Daily Star Online has approached Buckingham Palace for comment.