The threshold varies across different regions, from 25C in northern and western parts of the UK to 28C in south-eastern England.
A heatwave may be declared in London on Friday, where it has passed the 28C threshold for two days in a row.
On Thursday, Suffolk became the first area in the UK to officially enter a heatwave, after temperatures surpassed 27C for a third consecutive day.
Scotland is forecast to reach 28C on Friday, while Northern Ireland may see highs in excess of 25C. For England and Wales, maximum temperatures will range from 28C to 31C.
The heatwave is expected to peak on Saturday. This heat and humidity will also fuel some torrential downpours and thunderstorms.
A yellow weather warning for storms has been issued by the Met Office for parts of northern England, the Midlands and Wales from 15:00 BST on Saturday until 04:00 BST on Sunday.
Although these storms will be sporadic, they have the potential to bring localised flash flooding, large hail and lightning.
While heatwave conditions are likely to continue, some relief is anticipated on Sunday, when the south and east of England will dip to the high 20s. Elsewhere, cooler air will gradually sweep in from the west.
Temperatures will drop further into Monday – but could rise towards the heatwave threshold again in the latter half of next week.
