A UK record has been broken for the number of consecutive September days reaching 30C (86F).
A 30.2C reading in Northolt, west London, on Thursday means the mercury has reached at least 30C four days in a row.
The previous September record was three days – in 1898, 1906, 1911 and 2016.
The Met Office said that Thursday could also be the hottest day of the year so far, with a provisional 32.6C recording in Wisley in Surrey.
If confirmed, it will surpass 32.2C registered on two days in June in Chertsey, Surrey, and Coningsby, Lincolnshire.
Several other locations reported temperatures in excess of 30C on Thursday and the south-east of England could get 33C on Saturday, said BBC Weather forecaster Gareth Harvey.
"The heat is expected to last into Friday and for some the weekend as well, with heat slowly getting pushed further towards the south-east," he said.
The south-east of England could also get 31C on Sunday, he added, but further north will be cooler, with much of Scotland and Northern Ireland in the low-20Cs.
He said there was a growing chance of some thundery showers in the north and west this weekend as winds switch to a south-westerly direction and pull in cooler, fresher air from the Atlantic.
Temperatures are expected to fall off next week.
This week has, however, been a different story.
The mercury reached 30.2C in Whitechurch in Wales on Monday, 30.7C in Wiggonholt, West Sussex, on Tuesday, and 32C in London's Kew Gardens on Wednesday.
A further record could also be broken this week for the greatest number of September days where temperatures have reached 30C or more in the UK. The current record of five was set in 1911.
The month's hottest recorded day was 35.6C in Bawtry, South Yorkshire, in 1906.
The UK Health Security Agency has issued an amber warning until 9pm on Sunday in nearly every area of England, indicating that the effects of high temperatures could be felt across the whole health service.
A lower yellow warning is in place in the north-east of England, which is experiencing cooler temperatures.
Prolonged heat above 30C is a risk for older people and those with respiratory or cardiovascular diseases.
Councils have also been working to treat roads and stop them from melting in the heat, the Local Government Association said.
Heatwaves have become more frequent, more intense, and last longer because of human-induced climate change.
The world has already warmed by an average of 1.1C since the industrial era began and temperatures will keep rising unless governments around the world make steep cuts to greenhouse gas emissions.
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