Princess Kate broke royal protocol while delivering a major message that sent "shock around the planet", a royal author has revealed.

Royal author Robert Hardman has revealed a subtle change in the way the mother-of-three signed off a video message was actually a break in royal protocol.

In March this year, the Princess of Wales released a shocking video telling the world she had been diagnosed with cancer and was undergoing treatment.

Hardman spotted that the video, shared on social media, was attributed to "Catherine, The Princess of Wales", rather than the traditional "HRH The Princess of Wales".

Writing in his new book Charles III, serialised by the Daily Mail, he called this "a tiny point, but a note-worthy one in an institution bound to a correct way of doing things".

The royal author also expressed the surprising nature of the video, as the palace had so far been keeping her health scare under wraps.

In January, Kensington Palace released a statement confirming Princess Kate had abdominal surgery and would be out of public duties until at least Easter, but did not give any more information.

Months later, Kate herself released a video explaining her situation in a revelation that sent "shock around the planet", Hardman said.

He wrote: "The Princess appeared in a pre-recorded video message that created a genuine sense of shock around the planet. She was undergoing treatment for cancer."

Hardman went on to call Kensington Palace staff "assiduous in not giving details of the cancer, or of the treatment" before it was later revealed that she had been going through chemotherapy since February.

Kate made her first public appearance since the announcement at Trooping the Colour on June 15. Before the surprise appearance, she issued a statement thanking people for their kind messages.

The statement read: "I have been blown away by all the kind messages of support and encouragement over the last couple of months,' she began.

"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days."