The Met Office says "substantial" changes in working practices and daily routines will be required and there is a high risk of failure of heat-sensitive systems and equipment.
Britain's weather forecaster has issued its first-ever red "Extreme Heat" warning for parts of England on Monday and Tuesday when temperatures could reach record highs, triggering a "national emergency" alert level.
The highest ever recorded temperature in Britain was 38.7 degrees Celsius (102 Fahrenheit) recorded in Cambridge University Botanic Garden on July 25, 2019.
The Met Office is now forecasting temperatures of 40 degrees Celsius for the first time in Britain.
"Exceptional, perhaps record-breaking temperatures are likely early next week," Met Office Chief Meteorologist Paul Gundersen said on Thursday.
Gundersen is predicting a 50 percent chance temperatures top 40 degrees Celsius and 80 percent chance a new maximum temperature is reached.
"Nights are also likely to be exceptionally warm, especially in urban areas," he said in a statement. "This is likely to lead to widespread impacts on people and infrastructure."
Separately, Britain's Health Security Agency (UKHSA) raised the heat health warning to Level 4 for England for Monday and Tuesday.
READ MORE: 'We do expect it to worsen': Fresh heatwave grips Western Europe
On the Met Office website, a Level 4 red alert is defined as a national emergency and is used when a heatwave "is so severe and/or prolonged that its effects extend outside the health and social care system.
"At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups," the website says.
The Met Office said "substantial" changes in working practices and daily routines would be required and there was a high risk of failure of heat-sensitive systems and equipment, which could lead to localised loss of power, water or mobile phone services.
"It's harder to cope with these types of temperatures in the UK because we're just not used to them," Hannah Cloke, climate expert at the University of Reading, told Reuters, alluding to the country's generally temperate, damp climate.
"It's about that lived experience of the heat and we don't have the houses designed to keep cool, we don't have air conditioning, and our infrastructure is not built for the heat at all."
Much of Europe is baking in a heat wave that has pushed temperatures into the mid-40s Celsius in some regions, with wildfires raging across Portugal, Spain, France and Croatia on Thursday.
READ MORE: Officials warn of extreme fire risk as Europe swelters in June heatwave