Friday is the third hottest UK day on record as temperatures reached 37.8°C at Heathrow Airport, the Met Office has said.
It comes as the meteorological experts issue new warnings about climate change in their latest annual State of the UK Climate review.
The weekend's blistering temperatures are expected to be short-lived and thunderstorms have also been predicted.
Alongside mercury hitting 37.8°C (100°F) at London's Heathrow Airport, Kew Gardens in west London, had a high of 37.3°C (99.1°F).
Forecaster Simon Partridge said: "This not only makes it the warmest day of the year, it also makes it the third hottest UK day on record."
Earlier, Brighton & Hove City Council tweeted that it was "concerned about the number of people in the city" and stated that "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 on Friday.
Dorset Council said car parks in West Lulworth were full and urged visitors to head somewhere quieter.
In Kent, Thanet District Council said some of its most popular beaches were full.
Southern Rail also noted that its services were "busier than usual today due to the hot weather as people travel to and from their favourite destinations".
It urged people not to board a train if there was not space for them to travel safely.
The Met said that although a large amount of rain is unlikely, some places may experience heavy downpours and receive as much as 15-20 mm of rain in less than an hour.
The storms will be caused by hot air moving in from mainland Europe, some parts of which will be cooler than the UK.
Popular tourist spots on the continent including Ibiza, Lisbon and Berlin are expected to reach 33°C (91.4°F), 30°C (86°F) and 25°C (77°F) respectively.
This is compared to areas of Kent and Cambridgeshire, where the mercury will rise to around 33-34C (93.2F).
The thunderstorms will quickly be followed by cooler weather caused by a weather front moving in from the West, dashing any chances of a UK heatwave.
The Met said the front will result in temperature drops of up to 10C overnight with highs of 26°C (78.8°F) and 21C (69.8°F) in London and Manchester respectively, on Saturday.
This weekend's hot weather comes as the Met Office issues a fresh warning about climate change.
The meteorological experts have compiled their latest State of the UK Climate review, which shows how the country continues to warm, with 2019’s average temperature (1.1C) above long-term 1961-1990 levels.
“The climate statistics over time reveal an undeniable warming trend for the UK", said Dr Mark McCarthy, head of the Met Office’s National Climate Information Centre.
He added that changes in weather and the environment are "to varying degrees a consequence of both global climate change and natural variability in our climate.”
The short spike of hot weather comes at the end of a below average July temperature-wise.
The UK average temperature for July is currently on course to be just 14.1°C (57.4°F), one degree less than the 1981-2010 long-term average of 15.2°C (59.4°F).
Britain surpassed 100 percent of its average monthly rainfall and only experienced two thirds (66 percent) of the expected sunshine for an average July, a total of 113.4 hours, data from the Met shows.
“We’ve not seen a temperature anywhere above 30 so far or even with a three in it, that is quite unusual for July,” a Met spokesman said.