New details have emerged about a rare moment when Queen Elizabeth II cancelled all of her royal engagements, raising concern at the time over her wellbeing. It was an extraordinary move during her 70-year reign - and one that did not go unnoticed.

The unexpected decision was publicly attributed to a bout of the flu, but a new account by royal historian Robert Hardman suggests there was more behind it.

Speaking on the Daily Mail's Palace Authorised YouTube show, Mr Hardman said Buckingham Palace announced the late monarch had come down with flu when she had cancelled all engagements and taken to bed. The revelation comes as part of his research for his new book, Elizabeth II: In Private. In Public. The Inside Story.

According to Mr Hardman, the decision came in the aftermath of a particularly tense and high-stakes royal moment - the investiture of the then Prince Charles as Prince of Wales.

"The ceremony was going to be the coronation mark two', the expert explained. "It was a very tense moment. Only a few months later, the trouble started again in Northern Ireland."

He continued: "It was all over the world really - you just had the assassination of Martin Luther King and Bobby Kennedy in America. People were really nervous, worried about the direction the world was heading in."

The global backdrop, combined with growing unrest closer to home, created an atmosphere of heightened anxiety - something that did not go unnoticed within royal circles.

Mr Hardman said the pressure on the Queen and her family had been building steadily in the run-up to the investiture, adding that she was "really worried that something was going to happen."

He went on to explain that while the Queen had long accepted the personal risks associated with her role, the situation felt different when it came to her family.

"The Queen had always taken the view that if something happened to her, she'd live with it - die with it. It went with the territory. But this was the threat of terrorism against her son, his event and the family."

The weight of that concern appears to have led to her taking some time out from her busy royal schedule.

"Afterwards, Charles went off on a tour of Wales. The Queen went back to London to bed, cancelling all engagements for the week. Very, very unlike her. She was meant to be going to the Wimbledon Finals, had various garden parties, things to do. The whole lot was cancelled."

"The Palace said she was suffering from the flu - an odd thing to be suffering from in early July", Mr Hardman noted. "Someone very close to her team told me that it wasn't flu, it was nervous exhaustion.

"I don't think you could call it a full nervous breakdown, because she was back on duty just over a week later - but it was the nearest thing to a nervous breakdown."

Mr Hardman's account suggests that the decision may have been influenced by the immense pressure and strain surrounding the event, rather than solely her health.

The moment stood out precisely because it was so unusual. Throughout her reign, Queen Elizabeth II was known for her unwavering commitment to duty, rarely stepping back from public engagements even during periods of personal difficulty.

Cancelling an entire week of appearances - including high-profile events such as Wimbledon and royal garden parties, marked a significant departure from that pattern.