The Army did not seek approval from Kensington Palace before publishing its ticket page, it is understood.
The Army is set to remove a claim made on its website that the Princess of Wales is reviewing Trooping the Colour in June, the PA news agency understands.
Tickets were being sold on the official website for the June 8 military spectacle, advertising an appearance by Kate, as of 8.30pm on Tuesday.
The PA news agency understands the event details were published because of Kate’s role as Colonel of the Irish Guards, the regiment which is trooping its colour this year.
However, it is understood the Army did not seek approval from Kensington Palace before publishing the page, and the website is due to be updated.
It also names the King as attending the main Trooping ceremony on June 15 – also known as the Birthday Parade, because it marks the sovereign’s official anniversary – with bookings being taken for a ballot for tickets.
The announcement had caused confusion because there had been no official confirmation from Kensington Palace or Buckingham Palace.
Charles is receiving treatment for cancer and Kate is recovering from abdominal surgery, but it is understood planning for major royal events continues, with organisers remaining flexible in the event of possible changes.
The princess has been spotted being driven close to her home in Windsor following huge speculation on social media about the state of her health.
Only Kensington Palace, Kate’s official office, can announce her attendance at a royal event and confirmation is not expected until nearer the time.
Kate left hospital on January 29 and is recuperating at her Adelaide Cottage home close to Windsor Castle after undergoing abdominal surgery but is not expected to return to official duties until after Easter.
An image circulating online shows the princess, wearing sunglasses, sitting in the front seat of a car being driven by her mother, Carole Middleton.
Since his diagnosis, the King has postponed all public-facing duties but is continuing with behind-the-scenes work on his red boxes of state papers and some in-person meetings.
Charles was pictured earlier on Tuesday during a pre-Budget audience with Chancellor Jeremy Hunt at Buckingham Palace, ahead of the politician’s major financial statement on Wednesday.
