The Met Office reports that Monday is the hottest day of the year so far in the UK, with a temperature of 34.8C (95F) in Cambridge.
Millions of people are under a yellow heat health alert as a result of the higher temperatures that have only been recorded in central and southern England.
Some had a more precipitous morning, with intense downpours to the north, for which a yellow thunderstorm warning has since expired.
Forecasters said Monday would see the peak of the current heat spike, with temperatures due to ease as the week continued.
London, East Anglia and the home counties were forecast to experience the warmest weather.
Temperatures had already exceeded 30C in the capital by midday on Monday, with several sites hitting over 32C by 13:30 BST.
Various parts of London, Writtle in Essex, and Wisley in Surrey all reached 33C - and temperatures of 32C were also recorded in areas ranging from East Malling in Kent to Holbeach in Lincolnshire.
Yellow heat health alerts, issued by the UK Health Security Agency (UKHSA), are active in the East and West Midlands, East of England, South East and South West, North West and London.
They are due to expire by Wednesday morning.
Yellow alerts are issued during periods of hot weather that are only likely to affect those who are particularly vulnerable, for example the elderly, or those with existing health conditions.
The hottest day of 2024 before Monday was 30 July, when temperatures reached 32C in Kew Gardens and Heathrow - both in London.
This is only the eleventh year since 1961 that temperatures as high as 34.8C have been recorded, according to the Met Office's provisional figures.
Six of those occasions have been in the last 10 years, the forecaster added.
Many extreme weather events - including heatwaves - are becoming more frequent and intense as a result of climate change, experts say.
The highest temperature ever recorded in the UK was 40.3C at Coningsby, Lincolnshire in July 2022.
The Met Office recommends that people keep curtains closed during the peak of the day and drink lots of water.
It was confirmed earlier that a child had died after being pulled from a canal in Wolverhampton on Sunday.
A West Midlands Ambulance Service spokeswoman said crews had found a boy in a critical condition and that, "despite everyone's best efforts, the child could not be saved and he was confirmed dead at the scene".
The RAC urged drivers to check the coolant and oil levels in their vehicles as high temperatures increase the risk of vehicle breakdowns and cars overheating.
Meanwhile, National Rail also offered some advice, urging passengers travelling on Monday to check their journeys well in advance - though not because of the heat.
The Met Office "has issued yellow weather warnings for rain and thunderstorms in parts of Britain today," it said in a post on X, adding: "Please check your journeys before you travel with National Rail or your train operator."
Torrential downpours affected parts of Northern Ireland, Scotland and northern England earlier in the day.
"More than 15,000 lightning strikes have been recorded over the UK in the last 24 hours, mainly over Northern Ireland and northern Scotland," Ms Bishop, from the Met, said.
"Thundery showers are possible elsewhere in England this afternoon."
Thunderstorms are most frequent during the summer months as hot, humid air adds more energy to the atmosphere.