Britain’s sweltering heatwave is set to peak on Friday with temperatures of up to 34C (93.2F).
The Met Office said London and potentially parts of East Anglia are most likely to hit that level, making it the hottest day of the year so far.
Away from the capital and south east, high temperatures of 27°C to 30°C (80.6°F to 86°F) are expected across most of England and Wales, making a much of the UK warmer than parts of Jamaica and the Maldives.
Thursday was the hottest day of the year so far, at 29.5°C (85.1°F) checked in in Northolt in west London on Thursday.
Highs of 26.1°C (78.98°F) were recorded in Cardiff, 21.4°C (70.52°F) in Edinburgh and 20.6°C (69.08°F) in Derrylin in Northern Ireland on Thursday.
The UK Health Security Agency and the Met Office have issued a level 3 health alert for London, the east of England and the south east.
This alert follows Level 2 issued on Tuesday and confirms that the Met Office threshold temperatures for an alert will be reached in three regions from Friday.
A level 2 alert remains in place for the East Midlands and the South West.
While England and Wales will enjoy the heat on Friday, it will be cooler in Northern Ireland and Scotland, both of which will have rain.
Click here to see the weather forecast for your area
And since Friday will bring no respite for most of the UK, for the first time in its history, Royal Ascot will allow gentlemen to remove their ties, according to The Daily Telegraph.
The men attending the prestigious event are almost always expected to wear full morning suits – complete with top hats, waistcoats and ties – but a spokesperson said: “The fans of course must arrive compliant but jackets and ties may be removed throughout the day due to high temperatures.”
Some attributed the extreme heat to climate change.
Firefighters have also warned that extreme temperatures lead to an increased risk of fires.
London firefighters said they have already seen more than 1,430 grass and open ground fires across the city this year.
Charlie Pugsley, its assistant fire safety commissioner, said: “We want people to enjoy the good weather and do it safely.
“Barbecueing on dry grass is unwise and can easily cause a very serious fire – damaging the immediate area and endangering nearby properties.
“We are also urging people to think twice before having balcony barbecues.
“We’re not trying to take the fun out of the heat wave, but for the sake of our city – and our firefighters who have to work in sweltering temperatures to fight these fires – we’d really like people to follow our board advice. .
Tips for staying cool – including a Royal Navy tour
Firefighters also warned people not to risk their safety for a refreshing swim.
The LFB said there were nine accidental drownings in London last year.
The highest temperature recorded in the UK in June was 96°F (35.6°C) at Southampton Mayflower Park in June 1976, but forecasters do not expect that temperature to be exceeded this week.