After the UK experienced its third-hottest day ever recorded on Friday, the heat is forecast to give way to rain and cooler temperatures.
Temperatures hit a high of 37.8C (100F) at Heathrow Airport at 2.41pm on Friday and also reached 37.3C (99.1F) at Kew Gardens, in west London, the Met Office said.
The heat prompted huge numbers of people to flock to beaches and sparked fears about how social-distancing measures could be adhered to.
Brighton & Hove City Council said on Friday it was “concerned about the number of people in the city”, and stated the “large numbers make it impossible to maintain physical distancing”.
It said there was limited capacity on public transport and urged anyone who was not already there to avoid travelling to the city.
The near-record temperatures came on the day the Met Office warned the global climate crisis was having an “increasing impact” on the UK, with data showing an “undeniable warming trend”.
Thunderstorms also affected parts of western Scotland on Friday.
The Met Office also issued a yellow weather warning of downpours across parts of Scotland as “areas of heavy rain” broke out and moved northeastwards.
Despite yellow weather warnings being issued for large parts of east and south-east England - and some of north east England and Yorkshire and Humber, thunderstorms only moved across north east England late on Friday evening.
Forecaster Greg Dewhurst said they pushed their way north-eastwards across parts of Yorkshire, Lincolnshire and north east England - and out into the North Sea.
He said: “Another batch of heavy rain and thunderstorms made its way across west and south west Scotland. They pushed themselves eastwards through the evening.”
Beachgoers enjoy the sunshine and sea on what is now Britain's hottest day of the year so far, in Brighton, Friday 31 July (AP)
The Met Office has also issued a yellow rain warning, which runs on Saturday from 3am to 10am, for Shetland.
It has warned residents that “20-30mm of rain is likely quite widely, with some more intense thundery outbreaks possibly giving 10-15mm within an hour in a few places”.
The hottest day ever recorded in the UK was registered at 38.7C (101.7F) at Cambridge University Botanic Gardens last year. The second hottest day was on 10 August 2003, when temperatures rose to 38.5C (101.3F) in Faversham, Kent.
The Met Office said temperatures will be back closer to average on Saturday, but it will still be pretty warm in the South East.
Met Office forecaster Simon Partridge said “the weekend is going to be pretty good”, and added: “Both days are going to be mostly dry across the UK with sunny spells and some showers at times that may be out to the north and the west of the country.”
Temperatures in the South East on Saturday could reach 26C or 27C and it could feel a little bit fresher with highs of 23C or 24C on Sunday.
The RNLI and coastguards are also telling people not to take inflatables to the beach and urging them to be aware of the tide times, local hazards and potentially deadly rip currents.
The warning comes as seven people were rescued after being caught in a strong, fast-moving current in Wales on July 26, according to the RNLI - whose lifeguards dealt with more than 1,500 incidents involving rip currents last year.
Additional reporting by PA.