A Level 3 heat-health alert for London, the east of England and the south east is in place to help protect health services, the UK Health Security Agency (UKHSA) has said.
Agostinho Sousa of the UKHSA said the alert is to help protect the NHS, adding: “The Level 3 alert is operational and called ‘heatwave action’.
He said: "It is to inform our partners that they should prepare their services for possible increases in demand due to increases in temperatures.
“We also have a Level 2 that is currently active in the South West and East of England that is to inform our partners that they should prepare their services in case they need to enter into action in case we see an increase in temperatures.
“Right now the situation is stable, and we expect the temperatures to drop tomorrow," he told BBC Breakfast.
Parts of the UK are expected to sizzle in temperatures of 34C as the hot spell of weather reaches its sweltering peak on Friday.
The Met Office said London and potentially some spots in East Anglia are most likely to reach the 34C high, making it the hottest day of the year so far.
Away from the capital and the South East, 27-30C is expected across most of England and Wales.
A high of 29.5C was recorded at Northolt in west London on Thursday, surpassing the 2022 high of 28.2C recorded at Kew Gardens on Wednesday.
Highs of 26.1C were recorded in Cardiff, 21.4C was recorded in Edinburgh and 20.6C was recorded in Derrylin in Northern Ireland on Thursday.
While England and Wales will bask in the heat on Friday, it will be cooler in Northern Ireland and Scotland which will both be affected by rain.
Firefighters are warning that there is an increased risk of fires due to the heatwave.
London Fire Brigade (LFB) said it has already attended more than 1,430 grass and open land fires across the city this year.
This includes grassland, canal and riverbank vegetation, scrubland, parks, playgrounds, crops and woodland.
Temperatures are set to soar across the UK today
Jonathan Brady
London and some spots in East Anglia could see temperatures as high as 34C
Ben Birchall