The Met Office has issued its first red warning for extreme heat, warning of a “potentially very serious situation” in parts of England.
The UK Health Security Agency has increased its heat health warning from level three to level four – a “national emergency.”
Level four is reached only when a heatwave is “so severe and/or prolonged that its effects extend outside the health and social care system”, according to the UK Health Security Agency.
Illness and death may occur even among the fit and healthy, and “not just in high-risk groups.”
With the heat building beyond the weekend, here’s what you need to know about how high temperatures could rise and when the heat will be most intense.
How dangerous is this?
Hot weather can put a strain on the heart and lungs, with older people, those with pre-existing health conditions and young children particularly at risk.
It can cause dehydration, heat exhaustion or even heatstroke, and affect the ability to work or concentrate.
The hot conditions have already put a strain on ambulance services, which are on the highest level of alert.
The London Ambulance Service has urged the public for support by only calling 999 in the event of a life-threatening emergency, and advises to keep hydrated and stay out of the sun during the hottest periods of the day.
Given the unprecedented nature of the threat levels issued by the government and the Met Office, authorities expect public services and even infrastructure – which has not been designed to withstand extreme heat in the UK – to come under severe pressure.
Met Office spokesperson Grahame Madge said: “We’ve just issued a red warning for extreme heat for Monday and Tuesday which is the first such warning ever issued.
“The warning covers an area from London up to Manchester and then up to the Vale of York. This is potentially a very serious situation.”
How hot is it going to get and when?
At the moment, forecasters say there is an 80% chance of the mercury topping the UK’s record temperature of 38.7C (101.7F) set in Cambridge in 2019.
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The Met Office's Mr Madge said there is a 50% chance of temperatures reaching 40C somewhere in the UK, likely along the A1 corridor.
There will be highs of around 30C on Saturday and Sunday, before the mercury rises much further on Monday – potentially reaching 37C.
It is expected that Tuesday will be the peak of the heatwave and could see the heat reaching 40C.
From Wednesday, the heat will start to break down, with the current forecast introducing a band of rain with some heavy and thundery downpours in places.
Where is it going to be hottest?
The most likely areas areas are north of London and up to Lincolnshire.
According to the Met Office, Peterborough, Grantham, Sandy, Stevenage are other areas around the A1 corridor will feel the heat the most.
So, how can we stay safe?
Look out for those who may struggle to keep themselves cool and hydrated, particularly older people, those with underlying conditions and people who lie alone.
Those who do live alone are being encouraged to ask a friend or relative to phone to check they are not have having difficulties during extreme heat.
Stay cool indoors, by closing curtains in rooms that face the sun to keep indoor spaces cooler, and remember it might be cooler outdoors.
Drink plenty of fluids to stay hydrated and avoid too much alcohol.
Never leave anyone in a closed, parked vehicle, especially babies, young children or animals.
Try to keep out of the sun between 11am to 3pm, when the sun’s UV rays are strongest.
If you have to go out in the heat, try to walk in the shade, apply sunscreen and wear a wide-brimmed hat.
Avoid physical activity in the hottest parts of the day.
Carry water when travelling.
Check the latest weather forecast and temperature warnings.
While going for a swim can be a good way to cool down, people are warned to head for lifeguarded swimming sites, to remember that water is often much colder than it looks, not to go too far from shore or swim against currents, and to always take a friend when swimming.
Weather Presenter Sally Williams explains what heatwaves are and what you can do to stay cool