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The UK is on track to break its record for the hottest day of the year for two days running with temperatures forecast to soar up to 33C on Friday.
Hot and dry conditions are expected to continue with the weather on track to reach the low 30s in many areas, but it could surpass Thursday's record of 32.2C, the Met Office said.
Health alerts have already been issued across England by the UK Health Security Agency (UKHSA) and will remain in place until 09:00 on Monday as it warns of health risks to the wider population.
The heatwave could peak on Saturday at a possible high of 34C, with thundery showers forecast for North Wales and northwest England, and dry and hot conditions in the east.
According to the Met Office, by Friday afternoon many regions are also expected to pass the heatwave criteria - which means a temperature threshold is sustained for three consecutive days.
A heatwave could be declared in London on Friday where it has passed the threshold of 28C for two days in a row.
On Thursday, Suffolk became the first place in the UK to officially enter a heatwave after temperatures passed 27C for the third day in a row.
Some relief is forecast for Sunday, where the south and east will dip to the high 20s, while temperatures will cool to the mid 20s elsewhere.
Despite the record-breaking heat so far this year, temperatures are still below the June peak of 35.6C in 1976.
The heat that continues to build is due to an area of high pressure across the UK which draws in hot weather from other parts of Western Europe with windy conditions.
The UKHSA warned of "significant impacts" across health and social care services and a potential rise in deaths particularly among people with health conditions and those aged over 65.
Firefighters have also responded to more than 500 wildfires across England and Wales this year - a 717% surge on the same period in 2024, the National Fire Chiefs Council said.
They are urging the public to exercise caution when spending time outdoors in order to prevent further spikes.