The heatwave threshold will likely be met for some at the weekend before the week starts with an amber extreme heat warning, with temperatures potentially climbing up to or above 34C.
The Met Office warning is in place on Monday and Tuesday for London, the east and south-east of England, and parts of the south-west of England and Wales.
It flags potential health impacts for people vulnerable to extreme heat and heat-related issues for the wider population, as well as a potential increase in water safety incidents.
The warning, which differs from amber heat health alerts issued by the UK Health Security Agency (UKHSA), comes as another heatwave is expected to sweep parts of the UK during the weekend and early next week.
Areas in southern and eastern England are likely to meet the heatwave threshold during the weekend, and there is a 40% chance of exceeding the highest-ever temperature for June, which is 35.6C set in 1957 and 1976, the Met Office said.
Saturday has a potential high of 28C before temperatures rise again from Sunday.
Some areas in the south and south-east of England are forecast to reach 32C on that day and then push up or above 34C on Monday.
They are again likely to reach the mid-30s on Tuesday.
Some locations may see temperatures remain above 20C overnight on Monday and Tuesday, particularly in urban areas.
Amber heat health alerts are in force by the UKHSA for the east, south-east and south-west of England, as well as London, until 8pm on Tuesday.
Greg Wolverson, deputy chief forecaster at the Met Office, said: “While heatwave criteria will be met for some in the south and south-east of England over the weekend, with temperatures into the low 30s possible, the warmth will expand and intensify at the start of next week, which, coupled with high temperatures overnight, leads to potential impacts.
“Temperatures will peak around the mid-30s on Monday and Tuesday. The humidity will also mean this feels close, warmer and more uncomfortable for many.
“Tropical nights – where the temperature doesn’t drop below 20C – are also likely for some.”
The warmth may bring some thundery downpours for some on Monday and Tuesday, though these should be “fairly isolated”, the Met Office said.
The very high temperatures could continue across a smaller part of the country into the middle of the week, but this remains uncertain for now, the forecaster added.
At least 15 people died after getting into trouble in open water during a hot spell in May.
Samantha Hughes, national water safety partner at the Royal National Lifeboat Institution, said: “With warmer weather approaching, it’s important to remember that the water is still cold.
“Entering it unexpectedly can lead to cold water shock, causing a sudden increase in breathing and heart rate, which may trigger panic.”
After the heat health alerts were issued, the Health and Safety Executive (HSE) said employers must ensure their workers are protected while maintaining productivity during the heatwave.
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