The Prince of Wales said tackling climate change is “utterly essential” as the country swelters in “alarming” temperatures.

While speaking at an open-air event to mark his 70 years as Duke of Cornwall, Charles said national commitments to reach net zero have “never been more vitally important”.

The nation is experiencing an extreme heatwave, with thermometers passing 38C, resulting in school closures, train services being cut and ambulance crews facing rising numbers of 999 calls.

The mercury hit 38.1C in Santon Downham, Suffolk, by 4pm on Monday, making it the hottest day of the year, while temperatures topped 37C in a number of other places.

Charles has long campaigned on environmental issues and last autumn told Cop26, the UN climate change summit, the world has had enough of talking and commitments need to be put into practice.

Speaking at the end of a garden party in the grounds of Boconnoc House near Plymouth, the prince said: “If I may so say, those commitments around net zero have never been more vitally important as we all swelter under today’s alarming record temperatures across Britain and Europe.

“As I have tried to indicate for quite some time, the climate crisis really is a genuine emergency and tackling it is utterly essential – for Cornwall, the country and the rest of the world.”

Cop26 president Alok Sharma, who led last year’s landmark UN climate change summit in Glasgow, has indicated he may resign if the next prime minister is not fully committed to the net zero agenda.

He said in an interview with the Observer that while it was “absolutely a leadership issue”, some of the remaining candidates in the Tory leadership race have been only “lukewarm”.

Charles also spoke of his belief in the sustainable elements of Nansledan, an extension to the Cornish coastal town of Newquay on land owned by the Duchy of Cornwall, the private estate of property, investments and land that provide an income to the heir to the throne.

He told the guest: “In the development of Nansledan, as well as in the Duchy’s new, regenerative agricultural practices that will help us to meet, we hope, our Net Zero Carbon commitments, we have only been able to make what progress we have by working in partnership with our tenants, our suppliers and the people of Cornwall.”

In a lighter moment, Charles joked about touring a country agricultural show with its hard of hearing chairman during one of his many trips to the Cornwall during the past decades.

He said: “There have been so many memorable visits over all these years – especially to the Royal Cornwall Show – where, years ago, and in the days of the then chairman, Sir John Molesworth-St Aubyn, who was a wonderful character and extremely deaf, I remember asking him ‘if I could visit the Bee tent.’

“After walking half the way round the showground, following him, he brought me to the lavatory tent – the ‘Pee tent!'”

During the garden party, the couple mingled with individuals from all walks of Cornish life from Duchy of Cornwall tenant farmers to charity workers and members of the Armed Forces.

Earlier, the couple had visited the picturesque fishing village of Mousehole, near Penzance, where temperatures were around 10 degrees below the sweltering 38C in London and the south-east.

The duchess told locals “This is very cool here. It’s stifling in London,” and joked “I’m using my parasol. I think I’m going to take off like Mary Poppins.”

While Camilla kept the burning sun at bay, the prince remained cool in his sunglasses and suit – buttoned at the front.

Over the coming days, the couple will tour the county and neighbouring Devon to mark 70 years since Charles became the Duke of Cornwall on the Queen’s accession to the throne in 1952.