The Duke and Duchess of Cambridge have praised the selfless dedication of frontline NHS staff during a visit to Croydon.

William also urged the public to heed expert advice on social isolation as he described the health service and its workers as the "very best of our country and society".

He spoke after the couple visited an NHS 111 call centre in Croydon, the first engagement the royal family has publicly undertaken out and about in connection with the pandemic.

The unannounced visit took place on Thursday, but was kept secret so as to not put any additional pressure on the centre, avoid crowds gathering and not clash with the Queen's statement to the nation in which she urged the country to "work as one".

Afterwards, William said: "The last few weeks, and more recent days, have been understandably concerning with the continuing spread of coronavirus.

"But it's at times like this when we realise just how much the NHS represents the very best of our country and society - people from all backgrounds and walks of life with different experiences and skills, pulling together for the common good.

"Not only are NHS staff and emergency workers responding to the needs of the public, they - like the rest of us - are concerned about their families, friends and loved ones. They need our support as much as we need theirs.

"That is why Catherine and I were proud to visit staff working at NHS 111, to pass on our personal thanks, along with those of my grandmother and father, to staff working around the clock to provide care and advice to those that need it most. It was also brilliant to see the great online tools for those with mild symptoms or worries."

He added: "All of us have a part to play if we're going to protect the most vulnerable. That means acting on the latest expert advice, staying home if we or those we live with have symptoms, and avoiding non-essential contact to help reduce the spread of the virus."

During the visit Kate told the first call handler she spoke to: "It's amazing. You're doing such a great job bringing everyone together and providing that, the support system, for the whole public."

William added: "There's a lot of people out there who want to help. A lot of work is closing down elsewhere so people are going to want to come and volunteer, people want to help, people want to be there to support you guys, and everyone knows what a fantastic job you guys are doing."

The couple heard that the number of calls from the public had quadrupled since the crisis began and that the 999 service had just had the busiest three days in its history.

The visit saw the couple adhering to the latest protocols about dealing with the disease, with no handshakes, frequent hand-sanitising and a conscious effort to socially distance themselves from people.

Garrett Emmerson, the chief executive of the London Ambulance Service, said the duke suggested that royal visits might change as the virus progresses through the population.

Mr Emmerson said: "We are all learning new ways of behaving socially as a result of this, and indeed new ways of working. We were talking with the duke about potential future visits if we are in further stages of social distancing - we may be doing remote conferencing for events like this."

While the couple sanitised their hands twice during the visit, one member of staff forgot the new rules when she asked William and Kate for a photograph and without thinking reached out to shake their hands.

"Don't shake hands!" William remonstrated with a laugh as he brought down his other hand to cut her off.