The claim was made in a piece by royal historian Robert Lacey who has penned a book about the meltdown of the princes’ relationship.

Senior royals are funding an investigation into allegations of bullying by the Duchess of Sussex.

The Buckingham Palace probe is looking into claims Meghan ‘humiliated’ staff, drove two personal assistants out of the royal household and undermined the confidence of a third staff member.

Her former communications secretary Jason Knauf wrote in a leaked October 2018 email that the duchess ‘seems intent on always having someone in her sights’.

Mr Lacey wrote in the Daily Mail: ‘According to one of my sources, it’s been pointed to him that everyone has a difficult sister-in-law.

‘William’s response was to nod his head in sage acceptance.

‘Then suddenly he broke out angrily. “But look at the way that bloody woman treated my staff — merciless!”.’

The Cambridges and Sussexes in happier times

Duchess of Sussex Meghan Markle is at the centre of a bullying investigation

The historian said the Sussexes have ‘drawn up a 30-page dossier in justification of their treatment of staff’ in response to the bullying inquiry.

He said the document includes precise details of why certain employees were let go.

Harry and William have been at loggerheads ever since the 2018 staff row.

The Duke of Cambridge was dismayed at the Sussexes’ decision two years later to quit as working royals and elope to the US.

Harry and Meghan walk down the stairs of the iconic Opera House in Sydney

A number of events in recent months have damaged the former best friends’ bond even further.

In an interview with Oprah Winfrey, Harry described how he felt sorry for William and his dad Prince Charles, who he described as being ‘trapped’ within the system of the monarchy.

The brothers are due to come together in Kensington Palace on Thursday to unveil a statue to their late mother Diana.

Mr Lacey reported they were ‘at each other’s throats as fiercely as ever’ the last time they met at Prince Philip’s funeral 10 weeks ago.

Meghan Markle in an appearance on Good Morning America

William will see his younger brother on Thursday for Diana’s statue unveiling

He called on Harry and elder brother William to both admit their mistakes and reconcile.

He wrote: ‘How about you two fractious royal highnesses considering the possibility that both of you are right, and both of you are wrong? That the time has come for two over-indulged young men to grow up and forget their differences.

‘Get over it for your own sakes and for ours — and become again the brothers you’re supposed to be.’