The record for the highest daily minimum temperature in May has provisionally been broken for a third night in a row, the Met Office said.
In a post on X the forecaster said the temperature did not drop below 21.4C in Camborne, Cornwall overnight.
However, temperatures are now expected to cool significantly in parts of the UK after several days of record-breaking heat.
The Met Office said Tuesday was the hottest day in May on record for both England and Wales, with Kew Gardens provisionally reaching 35.1C and Cardiff Bute Park reaching 32.9C.
The Met said cloud and a developing easterly flow of wind on Wednesday would lead to a “reduction in temperature highs for many areas” – although south-west England may still see 32C-33C.
Highs of 32C are forecast for Thursday in London and the East Midlands, with temperatures on Friday potentially reaching 30C in London and East Anglia.
By Sunday, the warmest air is expected to have cleared away to the south with temperatures closer to, but mostly above, average for the time of year for the whole of the UK.
Met Office meteorologist Alex Burkill said the “exceedingly hot” temperatures were “exceptional”.
Mr Burkill added: “It really is an exceptionally warm or very hot spell at the moment.
“For any time of the year it’s hot, but for May in particular – it is still meteorological spring.
“In terms of how ground-breaking, how historic it is – it’s very similar to that first time that we reached 40C.
“The fact that we’ve exceeded the May temperature by such an amount really is extraordinary and quite worrying.”
The UK Health Security Agency (UKHSA) said amber heat-health alerts for the South West, South East, London, East and West Midlands and the East of England will remain in place until 5pm on Thursday, with yellow heat-health alerts for the North West and North East.
The Met Office said a climate attribution study published last summer by its scientists found that the chances of surpassing the May temperature record “have been increasing as our climate changes as a consequence of human greenhouse gas emissions”.
The study found that breaking the previous 32.8C May record was “around three times more likely now in our current climate than it would have been in a natural climate not impacted by greenhouse gas emissions”.
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