Temperatures have soared above 28C in some parts of the UK, making it the warmest April day in nearly 70 years.
The Met Office said temperatures reached 28.8C in Northolt, Greater London this afternoon.
That is just 0.6C shy of the hottest April day ever in the UK, when 29.4C was recorded in Camden, north London, in 1949.
The southeast of England will enjoy the best of the weather, while most of England and Wales should see the temperature rise into at least the low 20s.
Conditions will be mild across the UK, with Northern Ireland likely to push 19C in some parts and a peak of 20C possible in Aberdeenshire in Scotland, the forecaster said.
The hot weather far surpasses the average maximum temperature for April, which sits at 11.4C.
The summery spell comes as a result of warm air from the Azores, off the coast of Portugal, being dragged up towards Ireland and the UK by the combined efforts of an area of low pressure over the Atlantic and high pressure over western Europe.
Competitors in the London Marathon on Sunday can expect hot and humid conditions with a forecast of temperatures between 21C and 23C.
Marathon organisers are adding more water, ice and shower stations along the 42.16km (26.2-mile) route.
Meanwhile, Met Éireann has forecast a very mild couple of days here, but fresher from Sunday, with unsettled conditions next week.
Saturday looks likely to be the warmest day with temperatures reaching 19C in parts of the country.