The UK could see near record-breaking temperatures on Friday, with forecasters expecting it to be one of the hottest days ever.
People are being warned not to get "caught out" by soaring temperatures, as the country will be hotter than some of Europe's top holiday destinations, including Ibiza and Tenerife.
The mercury is set to rise to at least 37C (98.6F) in London and the South East today, on the second day of a heatwave that could last into next week.
Met Office forecaster Emma Salter said: “It will probably be one of the hottest days ever in the UK today, maybe in the top five. The hottest was 38.7C which was set last year.
“Today it will be the hottest day of the spell, with the heat staying above 35C for several days and lasting well into next week.
“There is likely to be three days in a row that temperatures will exceed 35C, and that’s only happened three time ever.
“London will see the peak, but it will be very hot throughout much of England and Wales.”
The Met Office said that after a slightly cooler week people could underestimate the heat, which could cause dehydration and sunburn, especially among the most vulnerable.
Grahame Madge, a Met Office spokesman, said: "Everyone needs to be careful during this heatwave, especially on Friday, where we are going to see a dramatic rise in temperature in some parts, exceeding the heatwave threshold.
"After a slightly cooler week, people should make sure they aren't caught out. If you need to travel, keep hydrated and apply sunscreen; the chance of sunburn and dehydration will be much higher."
It is unlikely that temperatures will surpass the 37.8C (100.04F) recorded in Heathrow on July 31, which was the hottest day of the year so far and the UK's third warmest ever, but it will still be unbearably hot at times.
The threshold for a heatwave is three days of temperatures over 25C (77F) across most of the nation and 28C (82.4F) in London.
Public Health England (PHE) has issued a heat-health warning, with people advised to stay cool indoors by closing curtains that face the sun and ensuring pets or children are not kept in vehicles.
Ishani Kar-Purkayastha, consultant in public health at Public Health England, said: "This summer, many of us are spending more time at home due to Covid-19.
"A lot of homes can overheat, so it's important we continue to check on older people and those with underlying health conditions, particularly if they're living alone and may be socially isolated."
Elderly people are among the most vulnerable to hot weather, with advice telling them to contact neighbours if they are living alone, to try to stay indoors during the afternoon and to carry a bottle of water when out.