Wednesday is officially the hottest day of the year so far, with people flocking to beaches and beauty spots.
Met Office forecasters said temperatures hit 31C at Heathrow Airport, 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.
People have been told not to leave hand sanitiser in hot cars as they could catch fire.
An alert issued by NHS Property Services and shared on the Fire Industry Association website, warned there have been “a number of reports of hand sanitiser being the cause of fires when left in vehicles in the hot weather”.
The heatwave is expected to break from Thursday evening, with a yellow weather warning for thunderstorms is in place for much of the UK from Thursday, covering Wales, Northern Ireland and parts 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.
Tuesday missed out on being the year’s hottest day, with a temperature of 28.6C recorded at Heathrow Airport and Kew Gardens.
On Wednesday afternoon, temperatures are expected to reach around 33C in parts of the UK, the Met Office tweeted.
The forecaster added it would be cooler across the far north of Scotland but hot and sunny across most of England and Wales.
Met Office meteorologist Alex Burkill said temperatures would edge towards 33C as the UK experiences “lots of hot and sunny weather through the next couple of days”.
He added the heat would be “quite widespread”, and even across parts of Scotland there could be “highs of 26 or 27”.
The highest UK maximum temperature recorded in June is currently 35.6C, set at Mayflower Park, Southampton on 28th June 1976.
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.
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.
Dr Lynn Thomas, medical director at St John Ambulance, said: “You could end up in the sun for longer than expected on what would normally be a quick journey, such as queuing to enter the supermarket, so you should be prepared to look after yourself and others.”
Warnings have been also issued about UV levels, which are going to be “exceptionally high” over the next couple of days.
Mr 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.”