Wednesday is officially the hottest day of the year so far, with people flocking to beaches and beauty spots.
The Met Office said the temperature hit 32.6C at Heathrow Airport at 14:26 BST on Wednesday, beating the previous record of 28.9C set at the end of May.
A level three heat-health alert has been set for parts of England, with advice to take extra care in the sun.
The heatwave is set to break from Thursday evening, with a yellow weather warning for thunderstorms in the UK.
The Met Office warning covers parts of the UK, including Wales, Northern Ireland and areas of Scotland.
A separate yellow weather warning for thunderstorms across most of the UK is in place from 12:00 BST on Friday to 06:00 BST on Saturday.
Scotland and Northern Ireland have been slightly cooler, with a high of 26.9C recorded in Charterhall in Berwickshire, and 21.8C in Derrylin, County Fermanagh. In Wales, meanwhile, temperatures peaked at 30C in Cardiff.
Tuesday missed out on being the UK’s hottest day this year, with a temperature of 28.6C recorded at Heathrow Airport and Kew Gardens.
The highest UK maximum temperature recorded in June is currently 35.6C, set at Mayflower Park, Southampton on 28 June 1976.
People have been taking advantage of the heatwave by heading to beaches and beauty spots.
Crowds of sunbathers have been pictured on beaches in Bournemouth and Brighton, while others have preferred to take a dip in the water to cool off.
The Met Office raised the level of its heat-health alert to three for the West Midlands and East Midlands on Wednesday, as health officials advised the most vulnerable – many of whom have been shielding during the 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.
St John Ambulance advised shoppers to be aware they could be forced to spend extra time in the sun as a result of social distancing measures and to be prepared.
Warnings have been also issued about UV levels, which are going to be “exceptionally high” over the next couple of days.
Met Office meteorologist Alex Burkill said these will reach eight across many places and nine across parts of Devon and Cornwall on Thursday.
“That’s about as high as it gets really in the UK.”
Meanwhile, NHS Property Services warned that hand sanitiser should not be left in hot cars, as it can catch fire in high temperatures.
However the organisation later clarified saying it now understood that risks associated with hand sanitisers in cars “only become apparent when in contact with a spark”.
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