Baking sunshine across the country saw the Met Office raise its heatwave alert to level three for only the second time ever.

Temperatures in London had topped 30C by mid-afternoon - and even Aberdeen was on a par with Rhodes and Tenerife at 26C.

On what is set to be the hottest day of the year so far, almost the whole country was basking in Mediterranean weather. Most places saw temperatures of over 25C, well above average, with only Northern Ireland and Western Scotland missing out on the sun.

With forecasters predicting it will stay sweltering tonight and tomorrow, the heatwave alert was raised to level three in London and the South East for the first time since 2006.

It is only the second level three alert since the classification was introduced after the burning summer of 2003, and calls for immediate “heatwave action”.

Health and social care staff will be told to make daily checks on people they judge to be at high risk. Hospitals and care homes will be instructed to keep the vulnerable in “cool rooms” kept below 26C.

Visting hours could be rescheduled to mornings and evenings to reduce heat levels in the afternoon.

Health services across the region have been told to look out for heat-related illnesses and to be particularly vigilant about the elderly and vulnerable.

Yvonne Doyle, the Government’s regional director of public health in the South East, said: “Everyone’s health can be affected by the heat, with greater risks of dehydration, sunburn, heat exhaustion and heatstroke.

“Those most at risk are the very young and the very old, and people who already have health conditions, especially heart and respiratory problems - the heat can make their symptoms worse.”

The London Ambulance Service said it had been treating large numbers of people for breathing problems, chest pains and fainting. Yesterday it took 21 per cent more calls than the previous Tuesday.

The Met Office has advised people to stay out of the sun in the middle of day and keep windows and curtains closed to prevent hot air from getting in. When outdoors, they said people should stay hydrated and move more slowly.

But others were making the most of the heat, with beaches crowded across the country. Tourism chiefs in Bournemouth claimed that 50,000 people flocked to the town’s beach today as sun-worshippers took advantage of the holiday weather that is unlikely to last until the weekend.

Thunderstorms were approaching the South West this afternoon and are due to bring rain bursts and an end to the highest temperatures by Friday. But a Met Office spokeswoman said that “horrible, sticky humidity” would persist until the middle of next week, before a return to more seasonal temperatures in the low twenties.

Chief Forecaster at the Met Office, Eddy Carroll, said: “Temperatures are likely to peak today and tomorrow, reaching 32 degrees before becoming less hot by the weekend as noticeably fresher conditions with the risk of showers pushes east across the country.”