The King has laughed and joked with guests at a Buckingham Palace garden party, while his younger son Harry was on his own at a nearby event in London

The King spoke warmly with many of the attendees as he joined other senior royal family members at Wednesday’s party, including asking Olympic swimmers how long they could hold their breath.

Meanwhile, Prince Harry arrived at St Paul’s Cathedral just two kilometres away for a service to mark the 10th anniversary of the Invictus Games he helped to found, without wife Meghan Markle.

Notably, Harry’s uncle and aunt – Charles Spencer and Lady Jane Fellowes, siblings of the late Princess Diana – were among the attendees at St Paul’s.

But the service was notable for the absence of any of the prince’s royal relatives, including his brother Prince William.

Prince Harry greets well-wishers as he leaves St Paul’s Cathedral. Photo: Getty

Harry had already acknowledged he would not see his father on his brief visit to Britain, due to the King’s “full program”.

The monarch returned to public-facing duties only last week following his cancer diagnosis.

There were about 8000 people at The Sovereign’s Garden Party on a sunny afternoon at Buckingham Palace. The King, wearing a grey suit and light blue waistcoat and tie, smiled to the gathered crowd as he entered the garden from the palace’s Carnarvon Room.

He emerged alongside the Queen, who wore a white silk dress by Fiona Clare and a hat by Philip Treacy, as well as a heart-shaped diamond brooch that belonged to the late Queen Elizabeth II.

They were joined by the Princess Royal, the Duke and Duchess of Edinburgh and the Duke and Duchess of Gloucester.

All the family members stood at the garden steps to observe the national anthem, before the King shook his top hat with joy when the crowd cheered loudly as the song ended.

The King and Queen then greeted sections of the crowd and groups of attendees separately. One attendee who spoke with him said “God bless you” while another remarked: “Good to see you looking well.”

Among those the King spoke to were Isabelle Thorpe, 23, and Kate Shortman, 22, who will compete for Britain in artistic swimming at the Olympic Games in Paris in July.

The athletes were nominated by Swim England to attend the garden party. Shortman said the event had left her feeling “very patriotic”.

“He (the King) did seem like he had a genuine interest and he said he was going to watch our event on TV in Paris, so that would be really cool. Hopefully he does!” she said.

Shortman added: “He was asking us about how long we could hold our breath, which is a very common question.”

Speaking to another guest, the Queen said she was “enjoying the sunshine” and joked with a woman that she had “brought the good weather with you”.

The King and Queen eventually reunited before spending some time in the Royal Tea Tent, and stopped to chat with more guests before returning inside the palace.

Anne, Edward and Sophie continued to mingle with guests after the couple had left.

The monarch will have another public outing this week when he meets military staff and their families at a training base for the Army’s Royal Engineers on Thursday.

Next Monday, he and the Prince of Wales will unite for a joint ceremony to hand over the command of Army Air Corps – Harry’s old army regiment – to William.