According to the Met Office, parts of southern England, including the wider Thames Valley region, could see highs of 37C to 38C between Tuesday and Thursday, with even overnight temperatures expected to remain above 20C in some areas.
The prolonged heat is likely to affect daily life significantly, from travel delays to increased risks for vulnerable residents.
An amber extreme heat warning has been issued from Monday through to Thursday, covering much of southern England.
Separate amber heat health alerts from the UK Health Security Agency (UKHSA) warn of a likely rise in deaths, particularly among older people and those with underlying health conditions, as well as increased strain on health and social care services.
Forecasters say there is “growing confidence” that the UK could surpass its current June temperature record of 35.6C, set in Southampton in 1976.
Meteorologist Simon Partridge described the conditions as highly unusual and urged residents to take precautions.
He said: “It’s a very unusual, very rare spell of very hot weather, so it’s one of those ones where people should take care, try to keep their houses as cool as possible, check in on those that are vulnerable, so the very young and very old, in particular, are more vulnerable to extreme heat.
“But everybody will feel this because it will be very hot and the overnight temperatures, in particular, will mean that people won’t be getting as much sleep, probably, as they would usually do, so it will affect everyone.”
The extreme conditions are expected to bring disruption across the transport network.
National Rail has urged passengers to check journeys in advance, allow extra time and carry water, while there are warnings that high temperatures could impact infrastructure including the risk of rails expanding and even tarmac softening on roads.
Residents are being advised to stay hydrated, avoid direct sun during peak hours between 11am and 3pm, and keep homes as cool as possible.
Extra care is also being urged around open water, after a number of fatalities during previous hot spells linked to cold water shock.
The heatwave follows an already warm start to the summer, with experts noting that extreme weather events are becoming more frequent due to climate change.
While temperatures are expected to ease slightly heading into the weekend, conditions will remain warm into next week, extending the impact across the region.
Across Europe, similarly extreme conditions are being recorded, with temperatures exceeding 40C in parts of France and nearing that level in Spain and Italy, prompting widespread warnings and emergency measures.
