Soaring temperatures across the UK will be followed by thunderstorms and torrential rain over coming days, forecasters have warned.
The Met Office said Thursday could become the hottest day of the year, with temperatures expected to climb in the Midlands and Wales.
But thunderstorms are expected to form across the west of the UK in the afternoon, bringing heavy rain, lighting and hail.
West London is forecast to see temperatures peak between 32C (89.6F) and 33C (91.4F), while parts of Wales could see the mercury rise to 34C (93.2F).
Should that happen, it would be the hottest June on record in Wales, surpassing the previous peak of 33.7C (92.7F) on June 18 2000.
Met Office meteorologist John Griffiths said the Midlands could see temperatures reach 32C (89.6F) on Thursday, while the Welsh Marches – the area along the Welsh border – could see temperatures of 34C (93.2F) “very locally”.
The good weather is expected to turn from 4pm on Thursday as thunderstorms form in the south west and south east of England, Wales, western Scotland and Northern Ireland.
The Met Office has issued a yellow weather alert for those areas until 9am on Friday, warning of disruption due to flooding, lightning strikes, hail and wind.
Between 30mm to 40mm (1.6ins) of rain could fall in less than two hours.
The storms are expected to continue from midday on Friday through to 6am on Saturday, with another yellow warning in place for the whole of the UK.
Areas hit by the storms could experience “torrential downpours” with between 30mm to 50mm (2ins) of rain falling in an hour.
Wednesday was recorded as the hottest day of the year so far, with the mercury reaching 32.6C (90.7F) at London’s Heathrow Airport at 2.46pm.
The good weather saw people flock to beaches in huge numbers despite social distancing measures remaining in place.
The Met Office raised its heat alert level to three, as health authorities encouraged those most vulnerable – many of whom have been shielding during lockdown – to protect themselves amid the “exceptionally hot weather forecast this week”.
Public Health England (PHE) said older people, those with underlying health conditions, and very young children were all more at risk from the higher temperatures.
The amber level three, which remains in place for the West and East Midlands, requires social and healthcare services to target specific actions at high-risk groups, according to the Met Office website.
People have been advised to keep cool and stay hydrated where possible.
The Met Office said UV levels were expected to remain at eight across many parts of the UK on Thursday.
Dr Michaela Hegglin, associate professor in atmospheric chemistry at the University of Reading’s Department of Meteorology, said Thursday could see some of the “highest UV levels ever recorded” in Britain.
She said: “This is because of a combination of factors. We are at the summer solstice, when the sun is almost directly overhead at one o’clock.
“UV levels this high are rare in the UK, so people with light skin should be very careful to avoid getting burnt.
“While UV is important for getting vitamin D and keeping us healthy, too much of it can cause skin cancer or eye cataracts.”
Emer O’Connell, consultant in public health at PHE, said it was important that people kept checking on the vulnerable, as many continued to spend more time at home due to coronavirus.
“You will need to do things differently this year, for example keeping in touch by phone,” she said.
“If you need to provide direct care to someone at risk from hot weather, follow Government guidance on how to do this safely.”
Shoppers have also been advised to be aware they could be forced to spend extra time in the sun as a result of social distancing measures.
And dog owners have been urged to avoid exercising their pets during the hottest part of the day.
Vets Now, a provider of emergency veterinary care, warned that the average survival rate of a dog diagnosed with heatstroke was 50%.
The RSPCA said it had received 330 calls in regard to animal welfare and the hot weather since March 23, adding it was expecting “hundreds more” as temperatures increase this week.
The public is also being reminded to take care around water throughout the summer, amid concerns people might be tempted to take a dip to cool down.
Thames Valley Police said on Wednesday officers had found a body following a search for a missing man who is believed to have got into difficulty in the water at Lulle Brook in Cookham, Berkshire, on Tuesday.
London Fire Brigade has warned people not to have barbecues on dry grass, not to drop cigarettes or matches, and not to leave rubbish such as glass bottles lying around amid a risk they could start fires.