The UK has recorded its hottest temperature of the year so far – 33.9C (93F) at Gravesend in Kent.
The warmest weather has been in east and south-east England, where Met Office health warnings are in place.
Temperatures elsewhere in the UK have been significantly lower, though it has been sunny across much of the country.
The Gravesend temperature is some way below the highest ever recorded in the UK – 38.5C (101F) at Faversham in Kent in August 2003.
Met Office meteorologist Charlie Powell said Wednesday’s top temperatures could be followed by thundery showers overnight and on Thursday.
He said showers could affect much of England and Wales on Thursday, while it would be a fine day in many parts of Scotland and Northern Ireland.
A series of lightning strikes have already been recorded on Wednesday evening in Hampshire, Surrey and west London.
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The Met Office has issued level three heat-health warnings for London and east and south-east England, as well as a less severe level two warning for the east Midlands.
The advice says people should stay out of the sun, drink plenty of fluids and keep their homes cool by shading and closing windows during the day, then opening windows at night.
Dr Thomas Waite, of Public Health England, urged people to remember that others can suffer in hot weather.
“Older people, those with underlying health conditions and young children may all feel the ill effects of heat over the coming days,” he said.
“We’re urging everyone to keep an eye on those you know who may be at risk this summer.
“If you’re able, ask if your friends, family or neighbours need any help.”
What is a heatwave?
Although there is no official definition of a “heatwave” in the UK, the term can be used to describe an extended period of hot weather relative to the expected conditions in an area at that time of year.
This is different to the Met Office’s alert levels, which trigger action from health professionals and emergency planners.
The criteria vary by region – for example, the daytime temperature threshold in north-east England is 28C, but in London it is 32C. The average threshold temperature is 30C by day and 15C overnight for at least two consecutive days.
Parts of the UK are currently at level three of four – four being a “national emergency” red alert, which is issued when a prolonged hot spell becomes severe.
Source: Met Office
The previous top temperature recorded in the UK in 2016 was 33.5C (92.3F) at Brize Norton in Oxfordshire on 19 July.