An NHS heatwave health alert has been issued for parts of England today as Brits prepare to bake in 34C heat.
The 'Level 3' warning is in force for London, the east and south-east of England.
Today is expected to be the hottest day of the year so far, with temperatures topping those in Jamaica.
The "extreme temperatures" have sparked Level 3 alerts, which ensure social and healthcare services take specific actions to look after high-risk groups.
Agostinho Sousa of the UK Health Security Agency (UKHSA) told BBC Breakfast the alert is to help protect the NHS.
He added: "The Level 3 alert is operational and called 'heatwave action'.
"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."
The Level 3 heat warning says: "There is a 90% probability of Heat-Health Alert criteria being met between 0000 on Friday and 0000 on Sunday in parts of England.
"There is high confidence for temperatures to rise significantly through the second half of this week, becoming hot or very hot in central and southern areas.
"The peak of the heat is expected on Friday, with daytime maxima likely to reach into the low to mid-30s of Celsius across large parts of central and southern England.
"Another warm night is expected in similar parts on Friday night before cooler and fresher conditions arrive during Saturday.
"There remains uncertainty in regards to the speed of progression of cooler conditions arriving from the north and west over the weekend.
"A slower outcome may allow for the very warm or hot conditions to persist across far southern areas on Saturday, before a more definitive return to more seasonable temperatures for all regions of England by Sunday."
A Level 3 alert is one step away from a Level 4 'National Emergency' heat warning.
The Met Office said a Level 4 warning will only be issued when a heatwave is so strong that it may cause death among fit and healthy people - not just high-risk groups.
A spokesman for the Met Office said: "The Heat-Health Alert service has five response levels (Level 0 – 4) based upon threshold maximum daytime and minimum night-time temperatures.
"Each alert level should trigger a series of actions which are detailed in the Heatwave Plan for England, from long-term planning out of season, to summer and heatwave preparedness and action, to a major national emergency.
"These thresholds vary by region, but an average threshold temperature is 30C by day and 15C overnight for at least two consecutive days."
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A Level 2 alert remains in place for the East Midlands and the south-west.
Forecasters said London and potentially some spots in East Anglia are most likely to reach 34C today, 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.
