Face of Nation : The mercury could climb to 34C (93F) in London, according to BBC Weather.
Friday was the previous warmest day of the year, with temperatures reaching 30C (86F) at Achnagart in the Highlands and 29.5C (85F) at Porthmadog in north Wales.
Central and eastern parts of the UK “will be hot and humid” on Saturday, BBC forecaster Gemma Plumb said, predicting temperatures would reach the low 30s.
Highs up to 34C (93F) were likely in south-east England, she said.
“There will also be a lot of sunshine through today and so the UV levels will be very high.”
A temperature of 27C (81F) was expected at the Glastonbury Festival in Somerset, with organisers giving away free sun cream and water to help combat the heat.
“However, it does look as though some heavy showers and thunderstorms will move into northern parts of the UK as the day progresses and into this evening,” Ms Plumb said.
On Sunday, it is expected to “feel fresher” across the UK, with winds from the west.
