Experts say record temperatures could bring “population-wide adverse health effects and danger-to-life conditions”.
They also warn of localised power and water outages and even mobile phone services being affected.
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Local forecasters @NNweather said red weather alerts are extremely rare adding that a “significant weather event” is expected.
A spokesman said: “Anyone planning any outdoor activities or events on Monday or Tuesday will need to consider their plans carefully given the level of heat expected.”
Met Office red warnings are the most serious and means it is “very likely” that there will be a risk to life.
An amber extreme heat alert remains in force for Sunday but from Monday the Met Office warns people can expect "exceptional hot spell on Monday and Tuesday leading to widespread impacts on people and infrastructure.”
A statement said: “Population-wide adverse health effects may be experienced, not limited to those most vulnerable to extreme heat, leading to serious illness or danger to life.
“Substantial changes in working practices and daily routines will be required. Nights are also likely to be exceptionally warm.
“There is a high risk of failure of heat-sensitive systems and equipment, potentially leading to localised loss of power and other essential services, such as water or mobile phone services
“Significantly more people visiting coastal areas, lakes and rivers, will lead to an increased risk of water safety incidents
“Delays on roads and road closures, along with delays and cancellations to rail and air travel, with significant welfare issues for those who experience even moderate delays.”
Some forecasters are predicting thermometers could hit 40°C in the UK for the first time, beating the UK’s hottest ever day 38.7°C (102°F) set in Cambridge three years ago.
The highest temperature recorded at the Met Office Northamptonshire weather station at Pistford was 36.1°C on July 31, 2020.
How to keep cool in extreme heat
Public Health Northamptonshire has these tips for safe during three days of extreme heat forecast for the county:
■ Look out for those who may struggle to keep themselves cool and hydrated
■ Stay hydrated, take water with you if you are out and about
■ Avoid travelling at peak times particularly with children, especially babies, or the elderly. Ensure you have enough water with you to cope with delays
■ Stay out of the sun when UV rays are strongest between 11am and 3pm
■ If you need to go out, stay in the shade, apply sunscreen and wear a wide-brimmed hat
■ Close curtains in rooms facing the sun. Remember, it could be cooler outdoors than indoors
■ Never leave anyone in a parked closed vehicle – especially not animals, children or babies
■ Avoid alcohol, it dehydrates you
■ Watch out around open water, unexpected cold or strong currents can catch out even experienced swimmers. Better to swim safely at one of the county’s organised events.