Temperatures are expected to hit 28C in some parts of the UK on what is set to be the warmest April day in nearly 70 years.
The South East will enjoy the best of the weather, while most of England and Wales will see temperatures rise into at least the low 20s, the Met Office said.
The hot weather is way above the average maximum temperature for April, which sits at 11.4C.
The warmest April day on record was 29.4C in 1949, but today's potential high of 28C would beat the 27.8C recorded in April 2001.
Meteorologist Alex Burkill said: "There's a fairly good chance of 28C, there's about a 60% chance.
"Quite widely we are going to see low 20s, and for many it will be a little warmer than Wednesday."
A high of 25.3C was recorded in St James' Park in London on Wednesday, making it the hottest day of the year so far.
The current 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 warm weather is expected to last until the weekend and on Saturday evening Northern Ireland and Wales could see thunderstorms, the Met Office said.
This will bring with it cooler conditions and temperatures will be back down to 16 degrees in London by Monday.
Competitors in the London Marathon on Sunday can expect hot and humid conditions with a forecast of between 21C and 23C, Mr Burkill said.
Temperatures could climb higher than the 22.7C recorded in 1997, the year of the hottest previous marathon.
"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.
"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.