Parts of the UK are expected to be hot at 34C (93.2F) as the hot weather reaches its peak on Friday.
The Met Office said London and possibly some places in East Anglia are likely to peak at 34C, making it the hottest day of the year.
Away from the capital and the south-east, 27-30°C is expected in most of England and Wales.
A high of 29.5C (85.1F) was recorded in Northolt in west London on Thursday, surpassing the 2022 high of 28.2C (82.76F) recorded in Kew Gardens on Wednesday.
Maximum temperatures 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.
While England and Wales will be basking in the heat on Friday, Northern Ireland and Scotland will be cooler and both countries will suffer from rain.
Meanwhile, firefighters warn that there is an increased risk of fires due to the intense heat.
The London Fire Brigade (LFB) said it had already visited more than 1,430 grass and open fires across the city this year.
This includes pastures, vegetation on canals and river banks, bushes, parks, playgrounds, crops and forests.
About 350 of these were in home gardens and the LFB said that with extended periods of hot weather, there is concern that this number will increase.
Over the past five years, London firefighters have been involved in almost 600 barbecue-related fires, 45 of which took place on private balconies.
Fire Brigade Assistant Commissioner Charlie Pugsley said: “We want people to enjoy the beautiful weather and do it safely.
“Barbecuing on dry grass is reckless and can easily lead to a really serious fire, causing damage to the surrounding area and risking nearby facilities.
“We also encourage people to think twice before having a barbecue on their balcony.
“Easier than you think, a balcony fire can spread to others, which can not only leave you homeless, but also relocate hundreds of your neighbors.
“We’re not trying to take away the fun of the heat, but for the sake of our city and our firefighters who have to work in sweltering temperatures to put out these fires, we really would like people to heed our advice. board.
“We ask the public to remain vigilant and call 999 as soon as they see any sign of smoldering grass.”
Firefighters are also warning residents not to risk their safety for a cooling bath.
Over the past five years, crews have been involved in almost 300 reports of a person in the water or in imminent danger of entering the water.
There were nine accidental drownings in London last year, according to the LFB, making it the third-highest in the UK.
The UK Health Safety Agency (UKHSA) and the Met Office have issued a Level 3 Heat and Health Warning for London, the East of England and the South East.
This alert follows the Level 2 alert issued on Tuesday and confirms that the threshold temperatures set by the Met Office for the alert will be reached in three regions from Friday.
The Level 2 Warning remains in place for the East Midlands and South West.
The Met Office has warned that Thursday night could be challenging sleep conditions, with temperatures expected to remain between mid-to-high teens.
Forecasters said it would be called a tropical night if it didn’t drop below 20°C (68°F) anywhere.
The UK’s highest recorded June temperature was 35.6°C at Southampton’s Mayflower Park in June 1976, and forecasters do not expect that high to be surpassed this week.