Prince William and Prince Harry are defying their mother's wish for them to "stay true" to each other by refusing to reconcile and put aside their differences. According to the royal expert, Princess Diana would remind her two children that under the scrutiny of the public, they would only have one another to turn to for support.

       Speaking to the ITV documentary 'Harry and William: What went wrong?', royal author Robert Lacey said: "Diana apparently would actually sit down and say to them, 'Don't forget, you two boys are all you've got.'

       "You must be true to each other. I want you to promise each other in front of me that you will always be true to each other."

       Relations between the two brothers have been strained for the last three years, with rumours of a rift beginning to circulate in autumn 2018.

       Mr Lacey details in his book 'Battle of Brothers' the fallout that occurred after Prince Harry's wife Meghan Markle was accused of bullying members of Kensington Palace staff - a claim the Duchess of Sussex has fiercely rejected.

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       According to Mr Lacey, this led to a fierce confrontation between the two brothers, and ultimately the decision to split their royal households.

       Harry and Meghan went on to establish their own charitable endeavour Sussex Royal and moved away from the Cambridges to their own home in Frogmore Cottage.

       The working relationship between the two brothers became increasingly strained after Harry and Meghan announced in January 2020 that they intended to step away from the Royal Family and relocate to North America.

       Since leaving the UK, the Duke and Duchess of Sussex have conducted a number of high-profile interviews where they have criticised members of the British monarchy.

       In their tell-all interview with Oprah Winfrey in March, Prince Harry characterised his relationship with William as "space".

       He said that they were on "different paths" and added that both his brother and father were "trapped" within the institution.

       Meghan Markle also touched on her relationship with Prince William's wife, revealing that Kate had made her cry in a dispute over flower dresses in the lead-up to the royal wedding. This story had been inaccurately reported by the British tabloid press.

       Other revelations in the interview included Buckingham Palace ignoring the couple's mental health and a senior member of the Royal Family voicing a racist remark about their unborn child.

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       The interview caused a PR crisis for Buckingham Palace and reportedly 'deeply upset' the Queen.

       Although Prince Harry and Prince William displayed a united front during Prince Philip's funeral in April, reconciliatory talks behind closed doors were reportedly unproductive.

       The two brothers recently reunited for the unveiling of a statue in Princess Diana's honour on July 1.

       The statue had been commissioned by the brothers in 2017, in order to commemorate what would have been Diana's 60th birthday.

       Both William and Harry appeared relaxed in each other's company, although the Duke of Sussex only stayed at Kensington Palace for around 90 minutes before departing.

       Princess Diana died in a car accident in 1997 at the age of 36. Her two sons were only 15 and 12 years old at the time of her death.