There will be "extraordinary and perhaps unprecedented heat" in parts of the UK on Thursday, forecasters say.
Temperatures could rise to a record-breaking 39 ° C in the London area, and even exceed 30 ° C in other parts of the UK, according to BBC Weather.
The heat wave follows Wednesday temperatures of 30 ° C in the southeast, with the highest in Writtle, Essex, at 33.5 ° C.
The current all-time high of 38.5 ° C in the UK was recorded in August 2003.
BBC weather is forecasted as "uncomfortably hot and humid" on Wednesday night, and temperatures in some southern English city centers remain at all night over 20 ° C.
In Scotland, temperatures hit around 30 ° C in the hottest areas on Thursday, including the Central Belt.
A weather front near Northern Ireland will at times keep it cooler with more clouds, whereas elsewhere in the UK there is a lot of hot sunshine.
Network Rail warns of interference, and the mentioned train speed limits can be introduced in Ireland areas where there was a risk of tipping tracks.
The police reiterated their warning to exercise caution in the open water when the bodies of three swimmers were recovered.  At around 4:30 pm, a man's body was pulled out of the Thames on Wednesday after a 47-year-old reportedly entered the water at Kingston on Tuesday night.
In the past, another body was probably that of a 23-year-old who had disappeared while swimming in the Thames in the Shadwell Basin on Tuesday was salvaged.
And in Gloucestershire, when Wiltshire police searched for a man in their twenties who was missing at Cotswold Water Park, a body was found on Tuesday night.
Inspector Stuart Simpson of the Metropolitan Police Marine Policing Unit said, "While the Thames sometimes looks attractive, especially in this hot weather, it remains very dangerous all year round.
" The first time the water enters the surface may appear warm, but further in it may be freezing cold and th There are often very strong undercurrents.
"The initial shock of cold water often causes people to drown beneath the surface."
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The hotspots on Wednesday were in the South and East of England, Writtle in Essex at 33.5 ° C, Heathrow (32.4 ° C) and St. James's Park in London (31.9 ° C).
Northern Ireland and Western Scotland were the coolest areas, peaking in the low 20s.
Predictors estimated on Thursday a 70% probability that the temperature could exceed the current all-time high of 38.5 ° C.
BBC Weather said weather conditions could reach 39 ° C in southern and eastern England.
The Met Office issued a yellow warning for thunderstorms in windrows from England and Scotland, which lasted from 3:00 pm CEST on Thursday to 4:00 am on Friday.