The UK is expected to edge towards its hottest ever July day, with the mercury due to soar above 30C (86F) on Tuesday.
Forecasters predict temperatures of up to 37C (96.8F) before the end of the week, surpassing the current record for a day in July – 36.7C (98.1F) at Heathrow Airport in 2015.
It comes as Public Health England renewed warnings about the heat, urging people to keep hydrated, find shade and take protection against the sun.
Met Office forecaster Marco Petagna said: “On Thursday we’ll see an east and west split with showers in the western parts of England but the eastern parts will look very hot with 36 to 37C (96.8F).”
He said there is a 60% chance Thursday’s weather might surpass the current July record of 36.7C and a 30% chance of going over the all-time UK temperature record of 38.5C (101.3F) which was recorded in Faversham, Kent, in August 2003.
The country’s new Prime Minister will be announced on Tuesday as temperatures creep towards expected highs of 34C (93.2F) in London, although thundery showers are expected to hit Bristol in the evening and into the early hours of Wednesday.
Despite the cloudy start to Tuesday in some parts, temperatures are still expected to be fairly high during the day.
It will not be wall-to-wall sunshine in Scotland, either, with Glasgow due to experience heavy showers on Tuesday.
Mr Petagna warned “anywhere across England could see some thundery showers”.
He added the coolest areas will be in western England.
He said: “The temperatures will still be in the 20Cs so still fairly sunny but this part of England will be the coolest out of the rest.”
In central and eastern parts of England, heatwave thresholds – which vary between 25C and 28C (82.4F) across the country, acting as a benchmark for determining whether the meteorological phenomenon has been reached – are expected to be met as high temperatures persist for three days or more.
The temperatures will remain high – thought to be around 24C (75.2F) – overnight into Wednesday, making it uncomfortable for some people to sleep.
Public Health England said the heat should prompt people to drink plenty of fluids, avoid excess alcohol and open windows when it feels cooler outside.
Nobody should ever be left in a closed, parked vehicle, especially infants, young children or animals, it added.
And it said people should avoid the sun between 11am to 3pm but, if they do go out, they should walk in the shade, apply sunscreen and wear a hat.
Other tips include avoiding physical exertion in the hottest parts of the day and wearing light, loose-fitting cotton clothes.
It came as welfare charity the Dogs Trust advised owners to not leave pets alone in a hot car seat even for a few minutes.
In August 2003, the UK experienced heatwave conditions lasting 10 days, the Met Office said.