The UK has recorded its hottest temperature of the year so far – 32.2C (89.96F) at Heathrow Airport.
The figure surpassed the previous record of 31.6C (88.9F) set at the same site in west London on Sunday.
It comes as the Met Office issued amber and yellow thunderstorms warnings for much of central and southern England, alongside its first-ever extreme heat warning, which covers the South West, southern parts of England, the West Midlands and Wales until midnight on Thursday.
Public Health England also has a Level 3 heat-health alert in place until the end of Friday, warning of potential heat exhaustion and increased risk to the elderly, young and clinically vulnerable.
Another Met Office heat warning has been applied to Northern Ireland from 8am on Wednesday until the end of Friday, when temperatures are predicted to drop.
Until then the heatwave is set to continue.
The mercury will remain above 31C (87.8F) during the week, with forecasters suggesting the next few days could be the hottest of the year yet.
Parts of the UK could reach 33C (91.4F) by Thursday, which is hotter than top European holiday destinations such as Marbella, Mykonos and Tenerife.
Meanwhile, an amber thunderstorm warning is in place for parts of southeastern England, including Kent, Essex and Cambridgeshire, between 4pm and 7pm this evening.
Forecasters warn of water spray, sudden flooding and lightning strikes, which could lead to power cuts, damage to buildings and delays to public transport.
Bad weather hit parts of Essex around 4pm, bringing flash flooding and hailstones measuring a few centimetres across.
Grahame Madge, spokesman for the Met Office, said: “The lightning frequency could be quite intense.
“The thunderstorms we’ve got developing are purely because of the heat so they’re not likely to affect the heat in anyway.”
The Environment Agency has issued 16 flood alerts – meaning flooding is possible – in parts of London, Nottinghamshire and Derbyshire.
All four of the home nations have individually recorded their hottest day of the year so far since the heatwave began at the weekend.
Northern Ireland had its warmest day on record, as temperatures hit 31.2C in Ballywatticock, County Down, at 3.40pm on Saturday.
The previous highest temperature was 30.8C, reached on 12 July 1983 and 30 June 1976.
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