The UK Health Security Agency issued the alert, which started at 10am today (26 May) and is in effect until 5pm on Thursday (28 May).
It covers the South East – raising concerns for older people, those with health conditions, and health and social care services.
Hot weather records have been shattered in the last few days, with Monday seeing 34.8 degrees Celsius recorded at Kew Gardens, the hottest May temperature on record – only for this to be surpassed again today (May 26) with 35 degrees Celsius also recorded at Kew.
Greg Dewhurst, a senior Met Office meteorologist, said: “We see these changes happening so much more dramatically.
“In the past, heatwaves built and built and built and built over days and days and days – these now just develop so quickly.”
The soaring temperatures have already caused disruption for some residents, with South East Water supplying bottled water to customers affected by outages and low pressure in Kent and Sussex.
502 South East Water customers have had issues including outages and low pressure.
Steve Benton, incident manager at South East Water, said: “We know water supply issues during hot weather are deeply frustrating and incredibly inconvenient, and we are working as quickly and safely as possible to resolve them.”
He attributed the problems to a sharp rise in demand, explaining: “As a result of the recent hot weather, we’re seeing increased demand across our network and we’re having to pump far more drinking water than usual.”
Beyond the immediate impact on infrastructure, health authorities are warning of more serious risks.
The UK Health Security Agency (UKHSA) says there could be “a rise in deaths, particularly among those ages 65 and over or with health conditions” alongside a likely increase in demand for all health and social care services.
Other potential impacts include overheating in care settings, workforce challenges and transport delays.
Age UK has advised older people to remain indoors between 11am and 3pm and take regular cold baths or showers.
Over the Bank Holiday weekend, thousands of people descended on Brighton’s seafront.
The Met Office confirmed that heatwave criteria were met in eight regions across England on Sunday, including Heathrow, Kew Gardens, and Writtle in Essex.
Wales and Northern Ireland also saw their highest temperatures of the year, with 27.4C recorded in Cardiff and 23.4C in Armagh.
In Surrey, Sunday night set a new record for the UK’s warmest May night, with temperatures at Kenley Airfield not dropping below 19.4C.
While temperatures are expected to gradually fall midweek, conditions will remain warm and mostly dry, with predicted highs of around 30C in parts of England and Wales through to Thursday.
