Highest September temperature since 1911 as 34.4C recorded
The UK's hottest day of the year so far - and the warmest September day since 1911 - has been recorded in Gravesend, Kent, where it reached 34.4C (93.9F).
The spell of warm Autumn weather is forecast to continue across parts of England until Friday.
Public Health England (PHE) and NHS England have both warned that the weather can pose a risk to health.
Elsewhere, there has been heavy rain and thunderstorms in parts of northern England and on the south coast.
A yellow warning for rain is in place for northern England, as well as for southern Scotland.
As much as 30mm (one inch) of thundery, torrential rain could cause flash flooding and travel disruption, the Met Office said.
Prestbury in Cheshire had 32.4mm of rain in an hour, the Met Office said, while Manchester City's Champions League clash with Borussia Monchengladbach has been postponed - with pictures showing huge amounts of water on the pitch.
The highest temperature of 2016 had previously been 34.1C, which was reached on 23 August at Faversham in Kent.
The all-time record for September of 35.6C (96.1F) was set in 1906, in Bawtry, South Yorkshire.
NHS England has declared a level-two heat alert, which means there is a high chance that an average temperature of 30C (86F) by day and 15C (59F) overnight will occur over the next two to three days.
These temperatures can have a "significant effect" on a person's health if they last for at least two days and the night in between, it said.
Dr Thomas Waite, from the extreme events team at PHE, said: "Think today about what you can do, and for those around you, to stay cool during the daytime and particularly at night.
"Much of the advice on beating the heat is common sense and for most people there's nothing to really worry about.
"But for some people, such as older people, those with underlying health conditions and those with young children, summer heat can bring real health risks."
Meanwhile, meteorologist Mark Wilson said there had been "a lot of rain in a very short space of time" in north-west England, along with "a lot" of thunder and lightning.
"There are some torrential downpours passing through parts of the country at the moment but they will ease," he said.
The forecaster said the band of rain and thunderstorms looked set to push eastwards into Yorkshire and up to Scotland over the course of the night.
Manchester Airport tweeted that some inbound flights had been affected because of the storm, but later added that it was "fully operational again".
Why is it so hot?
Jacob Cope, BBC Weather Centre Meteorologist
Hurricane Hermine, which hit Florida in early September, pushed large kinks into the jet stream - large atmospheric waves which lock our weather patterns in place. For Spain and Portugal, that meant temperatures rising to 10C above average last week.
A large area of high pressure centred over northern Europe has brought southerly winds, which have drawn this warm air northwards, reaching our shores today. And we have mainly clear skies across much of England, so we're topping it up ourselves.
We saw 31C in September in 1973, and in 1961, in Gatwick, the temperature recorded was 31.6C. It's very doubtful that we will break the all-time record though, as a 1906 heat wave brought September temperatures of 35.6C (96.08F).
The earlier high temperatures meant that Britain was hotter than forecasts for Madrid and Los Angeles.
Dr Waite added: "The hot weather won't make life difficult for all of us; indeed, many of us will make the most of it when the sun shines.
"But some people may not be able to adapt to the extra strain hot weather will put on their bodies and may feel the ill-effects.
"Each year we hear stories of people who have fallen seriously ill because, even though it's hotter, they may wear clothes which are too warm for hot weather, they may not drink enough or just try to do too much."
Dr Waite advised people to close curtains on windows that face the sun during the day, and to open windows once the sun is no longer on them to get a breeze.
He added that people should think about turning off electrical devices, as they can generate unwanted heat.