Prince William was forced to go it alone to a thank you tea for the NHS, after his wife came into contact with someone who later tested positive for coronavirus.
William and Kate were both meant to be at an NHS Big Tea at Buckingham Palace on Monday afternoon, to thank staff who have worked during the COVID-19 pandemic.
But the Duchess of Cambridge was forced to self-isolate after finding out one of her contacts had contracted coronavirus on Friday.
She will be in isolation for 10 days.
William does not have to isolate and so he continued with the planned engagements including a service in St Paul's Cathedral on Monday morning.
Sharing pictures of the event afterwards on social media, the Duke and Duchess of Cambridge's account read: "Today we paid tribute to the incredible work of NHS staff who have gone above and beyond to tackle the Covid-19 pandemic.
"Organised by @NHSCharitiesTogether, the NHS Big Tea saw communities come together for a moment of reflection and to thank NHS staff and volunteers, with the event at Buckingham Palace one of thousands taking place on the 5th July in homes, hospitals, schools and community spaces across the UK."
William was pictured under an umbrella at one point as scattered showers passed through central London, but appeared to remain in good spirits, laughing and joking with the guests.
He was shown talking animatedly to the guests in the garden of his grandmother's London residence.
Watch: Boris Johnson and Prince William among those celebrating the NHS with ceremony at St Paul's Cathedral
Before the event, Alexandra Heys, nurse ward manager in the respiratory high care unit at Gloucestershire Hospitals NHS Foundation Trust, said: "The special tea at Buckingham Palace means a lot to NHS staff who have been through so much over the last year, and I feel so honoured to be meeting the Duke and Duchess of Cambridge today.
"I know from personal experience that the past year has taken a huge toll on NHS staff who have worked incredibly hard to take care of patients during a challenging time.
"My team at the respiratory high care unit has cared for over 300 very sick Covid patients, all of whom required dedicated around-the-clock care.
"All NHS staff have gone above and beyond, providing additional care in difficult circumstances, including connecting isolated and very sick patients with their families in their most challenging moments.
"We have all been through a lot, so seeing the public get behind the NHS Big Tea on such as massive scale is an immense boost and really makes a difference."
William was a guest at the ceremony at St Paul's on Monday morning, alongside Boris Johnson, Dr Ashley Price, who treated the first coronavirus patients in the UK, NHS chief executive Sir Simon Stevens, and Labour leader Sir Keir Starmer and Liberal Democrat leader Sir Ed Davey.
There were also NHS staff who worked on COVID wards, volunteers and carers.
NHS Charities Together is the independent national charity partner of the NHS and an umbrella organisation made up of 240 NHS member charities based within hospitals, mental health trusts, ambulance trusts, community health trusts and health boards across the UK.
The Duke and Duchess of Cambridge became its joint patrons in December 2020.
Watch: Duchess of Cambridge has gone into self-isolation after COVID contact