Saturday is provisionally the hottest day of the year, the Met Office has said, with a high of 33.2C (92F) at London's Kew Gardens.
That beats the provisional 32.6C (91F) recorded on Thursday.
The UK has now sweltered through six straight September days of temperatures over 30C (86F) - the first time that's happened since records began.
Saturday is likely to be the peak of the heatwave, the Met Office said earlier this week.
And it may all be over soon, as there is an "increasing signal for a thundery breakdown for some later in the weekend", forecasters say.
The UK Health Security Agency has issued an amber heat health alert, meaning weather impacts are likely to be felt across the health service, with those aged above 65 or anyone with pre-existing respiratory or cardiovascular disease at greater risk.
It comes as more storm warnings have been issued across the UK.
The weather warning means some places could experience flash flooding, lightning strikes, hail or strong winds, with interruptions to road access and public transport a possibility.
The same warning has been issued for Sunday covering Northern Ireland, northern parts of England and Wales as well as southern Scotland until 11.59pm.
On Wednesday, the UK experienced its hottest September day since 2016, with 32C (89.6F) recorded at Kew Gardens in west London.
The hot weather follows a cool and wet summer for much of the UK.