The mercury continued inching its way up the barometer on Thursday, hitting 28.8C and making it the hottest day in April since 1949.
The south east enjoyed the best of the weather, with Northolt in Greater London hitting the almost 70 year high, while most of England and Wales saw the temperature rise into at least the low 20s.
Conditions were 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 Met Office said.
The 28.8C temperature in Northolt 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 Met Office 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 the UK by the combined efforts of an area of low pressure over the Atlantic and high pressure over western Europe.
The warmest April day on record was 29.4C, in 1949.
Earlier in the day meteorologist Alex Burkill had wagered: “There’s a fairly good chance of 28C, about a 60% chance.
“Quite widely we are going to see low 20s, and for many it will be a little warmer than Wednesday.”
Thursday is expected to be the hottest day of the warm spell, with weekend temperatures dipping slightly before showery outbreaks on Sunday.
Competitors in the London Marathon can expect hot and humid conditions with a forecast of between 21C and 23C, Burkill said.
“There could be a shower but it’s not very likely. It’s not great conditions for running. In fact if any showers do come they might be very welcome,” he said.
Some hayfever sufferers could be affected by high pollen counts, he added.
Burkill said: “For anyone who suffers from tree pollen they will probably be feeling the effects, but that’s only about 20% of hayfever sufferers. Grass pollen season comes later in summer.”