Prince Harry laid bare his struggles in an interview with Oprah. (Courtesy)

Prince Harry was "drinking to oblivion" just two days after the inquest into his mother Diana's death concluded, a royal author has claimed.

The Duke of Sussex was furious at being unable to pay a separate personal tribute to his mother after the inquest in April 2008, with the royal response instead covered by a statement from Prince Charles, claims Duncan Larcombe.

Mr Larcombe, author of Prince Harry : The Inside Story, said Harry and William were at a stag do for cousin Peter Phillips on the Isle of Wight just two days later when it became clear Harry was "not right".

While many factors contributed to the tragic events in Paris, the High Court had concluded the People’s Princess died in an accident.

Writing for the Mirror, Mr Larcombe said: "Something was not right with Harry. He was visibly rocking backwards and forwards on his feet.

"Under his arm was a piece of plastic pipe taped at one end by a large funnel. At one point Harry kneeled down, put the other end of the pipe in his mouth as a pals poured two pints of bitter down his neck.

"Even by Harry’s standards, this was a young royal drinking to oblivion."

Mr Larcombe said Harry told him how he spoke to Harry at length, trying to understand what was the matter with him.

He added: "Harry was furious at the official statement his ‘father’s people’ had issued on his behalf, welcoming the inquest findings.

An inquest ruled that Princess Diana died in an accident. (Courtesy)

"As far as he was concerned he’d been forced into silence, denied the chance of paying a personal tribute to his beloved mother.

"When I plucked up the courage to ask him why he had not simply instructed the men in grey suits to issue a statement he was happy with, his answer remains ingrained in my memory.

"'Because they don’t work for me, they work for my father,' Harry barked back."

The author said he can "vouch for" Harry's version of events about what his life was like that he gave to Oprah Winfrey for her new Apple TV+ documentary.

He added: "Most people thought he was just Harry being Harry, enjoying a drink, a party a good night out.

Harry in April 2008. (Courtesy)

"But in reality, so much of Harry’s infamous hell-raising had a very different meaning to the young Royal.

"Whatever the public’s current view of Harry, the once popular Royal did have to cope with a great deal before he met Meghan.

"Senior royals are horrified by his very public sessions in the psychiatrists chair.

"But they would do well to remember the role they played in letting their star act suffer in silence for so many years.

"Can they really blame Harry for wearing his heart on his sleeve, after all that’s the reason he held a special place in the hearts of the public since the age of 12."

A 12-year-old Harry walks at his mum's funeral in 1997. (Courtesy)

Harry has levelled strong criticisms at the royal family in recent days, following on from previous comments that he had felt "trapped".

The Duke of Sussex appeared in a new documentary series The Me You Can't See for Apple TV+ in which he made further damaging allegations against the firm.

Speaking to Oprah Winfrey, he accused the royal family of "total neglect" and claimed his dad Prince Charles allowed him to "suffer" as a child in the face of the media.

He further claimed the monarchy and press attempted to "smear" Meghan Markle in the lead up to their original tell-all chat with Ms Winfrey in March.

The interview is said to have upset the Queen and Prince Charles, and left Harry's relations with the family "hanging by a thread".

In the interview, Harry also alighted on his struggles coping with the death of his mother, and admitted using alcohol and drugs to cope with the pain.