Bank Holiday Monday was the hottest on record in late August, the Met Office said.
Temperatures hit 28.2C (82.8F) in Holbeach, southern Lincolnshire, forecaster Oli Claydon said, surpassing the previous record of 27.2C (81F) in East Bergholt, Suffolk, in 1984.
The hot weather is down to a “ridge of high pressure” bringing still conditions and sweltering sunshine, according to Mr Claydon.
In 1965, the August Bank Holiday was moved from the first Monday of the month to the last.
In southern England, temperatures should stay around the mid-20s on Tuesday, but moving into Wednesday a “front from the North West will bring a fresher feel to things”, the meteorologist said.
The cooling wave will bring temperatures down to the mid-teens as the front makes its way south-east – but showers are expected to be sparse.
There was some confusion over the record because of a “suspect” reading in Cambridgeshire in 1990 – where a temperature of 28.3C (82.9F) was noted.
But that number did not tally with other readings from the area, invalidating the result, Mr Claydon said.