According to the Met Office, temperatures are expected to soar to 24C in south east England – compared to a cooler maximum of 14C in Scotland's central belt.
In Scotland, the mercury will peak at 15C in Banff, Stirling and Perth, and many parts of the country will see some rain.
Met Office forecaster Alex Burkhill said it was a “possibility” that Wednesday’s temperatures could surpass the March record of 25.6C (78F), set in 1968 at Mepal in Cambridgeshire.
Temperatures are forecast to hit 24C (75.2F) to 25C (77F), with the highest figures expected around London.
The Met Office earlier tweeted there would be “exceptional warmth” and “massive” temperature contrasts.
Mr Burkill said: “The warmth is going to be very widespread across the bulk of England and Wales.
“Scotland will be cooler by tomorrow, so a little bit fresher.
“For most, it will be largely sunny, with a few bits of fair-weather cloud in England and Wales.
“In Scotland, it will be cloudier with a band of rain edging its way southwards.”
Wet weather is forecast for Easter Sunday, with the chance of sleet or snow in Scotland.
Tuesday saw Wales record its hottest day of the year so far, with the mercury hitting 21.3C (70F) in Cardiff.
Northern Ireland also experienced its warmest day of the year with temperatures reaching 18C (64.4F), while Scotland saw the mercury rise to 18.5C (65.3F) in Lossiemouth, Moray.