Temperatures could soar to 28C (82F) in some parts on Thursday, making it the warmest April day in nearly 70 years.
The south-east will enjoy the best of the weather, while the temperature across most of England and Wales should rise to at least the low 20s, the Met Office said.
Conditions will be mild across the UK, with Northern Ireland likely to push 19C (66F) in some parts, and a peak of 20C (68F) possible in Aberdeenshire in Scotland, the forecaster said.
The hot weather far surpasses the average maximum temperature for April, which is 11.4C (53F).
The summery spell is a result of warm air from the Azores, off the Portuguese coast, being dragged up towards the UK by a combination of low pressure over the Atlantic and high pressure over western Europe.
A high of 28C would beat the 2001 April high of 27.8C (82F), the Met Office said.
The warmest April day on record was 29.4C (85F) in 1949.
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 (78F) was recorded in St James' Park in London on Wednesday, making it the hottest day of the year so far.
Thursday is expected to be the hottest day of the warm spell, with weekend temperatures dipping slightly and showery outbreaks on Sunday.
London Marathon runners can expect hot and humid conditions with a forecast of between 21C (70F) and 23C (73F), Mr 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 hay fever sufferers could be affected by high pollen counts, he added.
Mr Burkill said: "For anyone who suffers from tree pollen they will probably be feeling the effects, but that's only about 20% of hay fever sufferers. Grass pollen season comes later in summer."