The Duchess of Edinburgh was absent from Wednesday night's banquet at Guildhall, sparking questions from royal fans on social media.
The Duke of Edinburgh made a solo appearance at the formal dinner with Emperor Naruhito as they continued their state visit in the UK.
Buckingham Palace previously shared in a press release that Sophie was scheduled to join her husband at the banquet, however, HELLO! understands that this was an error and it was only due to be the Duke of Edinburgh at the event.
HELLO! understands that the Duchess had a prior commitment which would have made the practicalities of attending the Guildhall banquet difficult.
According to the Court Circular, Sophie attended the Corps of Royal Electrical and Mechanical Engineers' Festival of Sport at The Prince Philip Barrack in Chippenham, Wiltshire in her role as Colonel-in-Chief.
It comes after the mother-of-two dazzled at the Buckingham Palace state banquet on Tuesday night, wearing an emerald green gown by Suzannah London with breathtaking jewels from the royal vaults.
For the first time, Sophie, 59, wore the Lotus Flower Tiara – a headpiece previously worn by the Princess of Wales and Princess Margaret.
The diamond floral-shaped jewels once belonged to the Queen Mother and was originally a necklace from Garrard.
The Duchess was also loaned the late Queen Elizabeth's pear-shaped diamond drop demi-parure with matching earrings from the suite.
Earlier in the day, Prince Edward and Sophie joined the King and Queen, and the Prince of Wales, as the Emperor and Empress were given a tour of a special exhibition in the Picture Gallery at Buckingham Palace, displaying items from the Royal Collection relating to Japan.
State Banquet jewels
Queen Camilla wowed in an embellished white Fiona Clare gown with the Burmese Ruby and Diamond Tiara and the late Queen's diamond festoon necklace.
And in another royal first, the Queen debuted King Charles' Royal Family Order, a miniature portrait of the monarch traditionally worn by female members of the royal family with a silk ribbon.
Traditionally the orders were made from ivory, but in line with modern times and the King's views on conservation, his royal family order is made from polymin - a synthetic translucent treated plastic sheet.
The portrait created by Elizabeth Meek shows the King dressed in military uniform and wearing his medals. The diamonds surrounding the portrait were already owned by the Royal Collection.