The Met Office has issued a weather warning for thunderstorms – the first such advisory since the service was introduced.
Forecasters said storms may develop and bring torrential rain, hail and lightning to places in south-west England and Wales on Sunday.
The yellow warning runs from 6am to 10pm, while the heatwave is set to continue for the rest of the weekend.
Temperatures reached a high of 29.5C (85F) on Saturday and the Met Office said there were no reports of rain, while Otterbourne in Hampshire was the UK hotspot.
Sunday will continue in a similar vein with parts of the Midlands and London reaching up to 30C (86F) with higher levels of humidity than in previous days, forecasters have said.