Only the North East and North West are exempt from the UK Health Security Agency’s (UKHSA) latest warning, which comes into force at 12pm on Wednesday and expires at 6pm on Sunday.
The alert indicates that people with pre-existing health conditions, and those aged over 65, could be at higher risk.
Forecasters say the East of England is likely to see the highest temperatures, which wouldn’t be far off the June record of 35.6C (90F) set in 1976.
According to the Met Office, it will get progressively warmer as the week progresses – with the heat peaking on Sunday.
Deputy chief meteorologist Dan Holley said thundery showers may be possible heading into Saturday morning, with “tropical nights” a possibility as parts of the UK approach heatwave territory.
The forecast means we are likely to see the hottest day of the year so far – eclipsing the 29.4C (85F) recorded last Friday in Suffolk.
In a delicious twist, ice cream makers have said “it’s their Christmas time”, with some making fresh supplies around the clock.
But the London Fire Brigade has warned that the hot weather, which follows one of the driest springs on record, also “greatly increases” the risk of wildfires that can spread rapidly.
A heatwave is officially declared when temperatures exceed a certain level for three days in a row – and thresholds vary from 25C to 28C (77F to 82F) across the UK.
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