Temperatures could hit 23C in Portsmouth tomorrow, making it the warmest day of the year so far and one of the hottest April days on record.
The Met Office has said the mercury will reach close to 25C tomorrow afternoon.
Forecasters are predicting low winds although it will feel more humid in the morning.
The warmest April day on record was 29.4C (85F) in 1949.
Today the south east will enjoy the best of the weather, while most of England and Wales should see the temperature rise into 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 sits at 11.4C (53F).