The decision by Prince Harry and Meghan Markle to agree to a highly emotional TV interview about their treatment at the hands of the press could open them up to further damaging headlines, according to PR experts and royal watchers.

In the ITV documentary, Harry & Meghan: An African Journey, Harry appeared to give credence to long-standing rumours of a rift with William when he admitted the brothers had “good days and bad days” and that they were following different paths.

As fallout over the extraordinary interviews continued, the PR expert Mark Borkowski said the couple’s participation in Sunday’s “heart-on-sleeve” programme had broken all the normal royal family rules of “never complain, never explain”.

“You should just say, ‘that’s off limits’. But he made a story,” said Borkowski, who called the documentary a “complete and utter disaster”.

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“The population is split. I think younger people get it and are more attuned with it. But the rump of people who are monarchists still probably go out and buy the Daily Express. Those are the people he is not convincing.”

With William destined to be king and Harry sixth in line to the throne behind his brother’s three children, their future paths will be different. But Harry’s comments were overwhelmingly interpreted as concrete evidence of a rift, in a blizzard of headlines.

“You allow the intrusive lens into your life, and you are out of control. No matter how much editorial control they had with their mate, Tom Bradby, he’s caused chaos,” Borkowski said.

The decision by Harry and Meghan to speak out so openly about their struggles, he said, had a hint of the A-list celebrity Hollywood PR strategy. But the rules were “totally different for celebrities and royals”.

He said: “The royal family are a publicly-funded heritage business, and as anybody that has entered in to [it] has discovered, the rules go out of the window. [Celebrities] don’t have the same responsibility, don’t have the love of the nation to deal with.”

While celebrities could demand “approved quotes, approved pictures, playing hardball, controlling photo opportunities”, he said, “with the palace there is an uneasy relationship with the media and the public”.

The royal biographer Penny Junor said: “My instinct is it is best not to tell the entire world about your emotional state. I suppose in a way, it is a generational thing, and everyone will have a different view on it.”

The documentary was filmed during the couple’s recent 10-day tour of southern Africa, a highly successful “textbook royal” tour which was then overshadowed by the legal action by Meghan against the Mail on Sunday, over allegations that it printed extracts of a private letter from her to her estranged father, Thomas Markle.

Harry also issued a furious statement railing against the tabloid press, whom he accused of a “ruthless” campaign against his wife.

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Meghan’s almost tearful gratitude when she was asked in the documentary if she was “OK” and about her struggle with the intensity of tabloid interest, was seen to have parallels with Martin Bashir’s Panorama interview with Diana, Princess of Wales.

“I had a great feeling of deja vu watching Meghan with her face beginning to crumple,” said Junor. She said the film raised concerns over the mental wellbeing of the couple. “I don’t doubt they are finding it very difficult. Fundamentally, all these negative stories, I guess, have taken their toll.”

The couple have announced they will take six weeks off from royal duties, during which they are expected to travel to the US.

But the documentary had served only to give everybody more headlines – and of the wrong sort, Junor believed.

“It just feeds the media machine, and that is the one thing Harry really hates. This whole documentary has exposed him. In a way it is the very reverse of what he has said he wants for him and his family, namely, privacy.”