Soaring temperatures are expected to hit at least 30C in south-east London and north Kent today (July 17).
Meteorologists expect today to be the hottest of the year, breaking the 30C (86F) barrier for the first time.
And the mercury could creep still further tomorrow, with temperatures as high as 32C as a belt of Iberian weather hits UK shores.
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.
Meteorologist Krista Mitchell, with the Exeter-based Met Office, said: "Things are certainly heating up today and again tomorrow."
The Met Office said the highest temperature of the year in the UK was 28.7C on July 4.
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 forecaster Ms Mitchell said: "Much of the country can expect thunder from Friday into Saturday.
"We can expect some quite torrential showers over a large swathe of the country. "
PHE 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 is expected to remain warm into next week.
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."