The mercury hit 28.7C in Northolt, west London, on Monday afternoon - just nudging past the previous record, set in 1995, when temperatures peaked on the Saturday at 28.6C.
The soaring temperatures also make it the warmest early May bank holiday Monday since it was first introduced in 1978.
The south-east and central southern England experienced the most heat, with the majority of the UK seeing temperatures between 23C and 27C.
In Scotland the highest temperature was recorded at Charterhall, near Greenlaw, in Berwickshire.
The mercury there soared to 23.1C – narrowly missing the previous record of 23.3C north of the border.
Edinburgh was not far behind at 22C, while Glasgow reached 20C.
But not everyone in the country has been basking in tropical heat.
Persistent cloud saw some Scots left shivering, with temperatures in single figures.
Thermometers did not go over 7.5C on the Bealach Na Ba, near Applecross, in the northwest Highlands.
A high of just 11C was recorded in Stornoway and 12C in Aberdeen.
The maximum average temperatures for May are 13C in the north and 16C in the south.
The weekend’s hot conditions have been created by a mixture of low pressure, light winds and consistent sunshine, according to weather forecasters.
Helen Chivers, from the Met Office, said: “We had a large area of high pressure dominating the weather across the UK for a good few days, so that means we’ve had settled conditions.
“Then we’ve had a feed of air from the continent, from the south, which is a warm direction for the UK. It’s those two things combined that have brought the hot spell.”
But the warmth is not expected to last.
Forecasters say conditions are set to become fresher, with a chance of showers for the rest of the week.
Temperatures are expected to drop slightly from Tuesday onwards as the warm jet stream moves diagonally south east across the UK and is replaced by cooler air.
The weather is set to become more mixed as the month progresses.