Britons were sent scampering for shade as the temperature soared to the highest level of the year today.
The mercury soared as temperatures peaked at 29.1C (84.4F) in London, while much of Scotland, Wales and Northern Ireland also saw temperatures in the mid-to-late 20s.
The Met Office said the conditions at Hampton near Richmond passed 2014's previous high of 28.7C, set two weeks ago.
But with an Iberian weather pattern expected to descend on the UK overnight, forecasters have warned tomorrow could get even hotter.
Forecaster Dan Williams said: "We would expect to get into the early 30Cs tomorrow so this is likely to be a short-lived record."
The top temperature in Wales today was 25.1C at Hawarden Airport, with highs of 22.8C and 23.2C in Scotland (Fife) and Northern Ireland (County Down) respectively.
That could climb a couple of degrees tomorrow as the so-called "Spanish plume" sweeps across the South.
It can lead to warm conditions and heavy showers or thunderstorms.
The hot, humid conditions prompted Public Health England (PHE) to warn of potential dangers, stating that heatwave conditions could have a "significant effect on health".
But those planning beach breaks or summer weddings this weekend may be wise to pack umbrellas, with the prospect of thunder and heavy showers overnight into Saturday.
It has prompted the Met Office to issue a severe weather warning for much of the country on Saturday, with the potential for localised flooding.
Exeter-based meteorologist Mr Williams said: "We could see some heavy showers in the South West tonight moving into Wales and the west of England tomorrow."
Malcolm Bell, board member of the Devon and Cornwall Business Council, said talk of a heatwave had been a shot in the arm for tourism in the South West.
He said: "There have been health warnings and talk of 30C temperatures keeping people inside, but it has had the opposite effect down here because people have been coming to the beaches and the coastal spots for the sea breeze.
"I was down on Perranporth (a long beach near Newquay in north Cornwall) at 6pm yesterday and the place was packed.
"It can be a real draw for people to escape the heat of the city.
"Even if people don't want to go swimming, plenty are happy for a dip in the sea or to be by the coastline to cool off a bit, so I'm sure this weather is doing well for the economy down here, as you'd expect."
Health warnings have already been issued, while motorists have been warned to take care as roads become sticky under the weight of increased traffic on the UK's sweltering roads.
AA spokesman Mark Spowage said: "It's going to be a hot and sticky time on the roads, literally in some places, as these temperatures can soften the road surface and make them uneven.
"Changing conditions on Saturday will bring torrential thundery showers in places.
"While they are likely to be short-lived, water will quickly run off causing localised flash flooding as well as making the roads pretty slippery.
"Keep your speed down and don't risk driving through flood water."
Public Health England said people should consider staying out of the sun during the hottest part of the day, drink plenty of fluids and wear sun cream that is at least factor 15.
It has also asked people to be aware of children and the elderly, to ensure they are not suffering because of the heat.
People should never be left in closed, parked cars, especially infants, young children or animals, a spokesman added.
Pet owners were also advised to keep their animals well-hydrated to avoid the risk of them developing heatstroke.
Forecasters said Sunday should bring fresher conditions but the south of the country was expected to remain warm into next week.
The Environment Agency said there was a risk of localised flooding over the weekend as a result of intense rainfall.
Craig Woolhouse, Environment Agency director of flood incident management, said: "Intense heavy rainfall this weekend brings a surface water flood risk for England on Saturday.
"Flooding can happen very quickly and the public are urged to check local weather forecasts and the Environment Agency web pages for information on a regular basis.
"We are monitoring the situation closely and supporting local authorities, who will respond to any reports of surface water flooding."
RAC spokeswoman Sarah Rice said: "While the weather is set to be hot yet unpredictable - and even stormy - we're not expecting this to significantly impact traffic on the roads this weekend as it comes a couple of days before the schools break up and so traffic volumes shouldn't be out of the ordinary."
Environment Minister Dan Rogerson said: " The Environment Agency is working closely with local authorities to prepare for any localised flooding. Our priority is public safety. I urge people to be prepared and act on the advice from the Environment Agency and local emergency services."