King Charles was pictured for the first time amid the outcry over the Mother's Day photograph of the Princess of Wales.
The monarch, 75, was seen waving from his car as he was driven through Central London on Wednesday, in a video shared by MailOnline.
The King is currently going through cancer treatment, having been diagnosed in February.
The King also met with Baroness Scotland of Asthal, the secretary-general of the Commonwealth after he had to miss the annual Commonwealth Service on Monday.
He delivered a pre-recorded virtual address at the Commonwealth Day service, which was attended by Queen Camilla, the Prince of Wales, the Princess Royal, the Duke and Duchess of Edinburgh and the Duke of Kent.
Last week, Charles, who has postponed most of his public-facing duties amid treatment, carried out a number of in-person engagements last week.
The King sat down with Chancellor of the Exchequer, Jeremy Hunt, in the palace's private audience room ahead of Budget day.
He also attended audiences with ambassadors and spoke with Canadian prime minister Justin Trudeau via a videocall from his desk.
The sighting of Charles came as his wife Camilla stepped out at Cheltenham Festival. See her arrival at the racecourse in the clip below...
The Queen, 76, looked elegant in a Anna Valentine ensemble – brown suede skirt, matching leopard print shirt and a dark green cashmere coat, with a wide brimmed Lock & Co hat.
Her Majesty, who is an honorary member of the Jockey Club which runs the major sporting event, was due to present the trophy to the winning owner, jockey, and trainer of The Queen Mother Champion Steeple Chase.
Joining the Queen at the racecourse in Gloucestershire were Camilla's children, Tom Parker Bowles and Laura Lopes, and Laura's husband Harry, alongside the Queen's nephew Sir Ben Elliot.
Mike and Zara Tindall, Princess Eugenie and Jack Brooksbank were also among the guests in the royal box.
Zara, 42, who is a racecourse director at Cheltenham, put on a chic display in a navy pinstripe suit with burgundy accessories, as she shared a kiss with her husband and former rugby star Mike.
Meanwhile, Princess Eugenie, 33, who wowed in a white belted coat over a knitted dress with brown boots, was seen sharing a giggle with her cousin Zara as they arrived for the second day of the Festival.
The Princess Royal, 73, who had a successful equestrian career in her youth and is a regular at Cheltenham, was pictured speaking with Queen Camilla's first husband, Andrew Parker Bowles, and winning jockey Paul Townend.
Princess Anne donned a tweed button-up coat with a camel trilby hat and a pop of red lipstick.