The Met Office has issued eight yellow weather warnings for Sunday and Monday.
On Sunday evening, the East Midlands, North West, South West, Wales, the West Midlands, and Yorkshire and the Humber could see a deluge.
The Met Office is warning of “heavy, thundery downpours” across these areas, with spray and flooding possibly leading to road closures and difficult driving conditions.
There is a “small chance” of power cuts and that buildings will be damaged.
Bus and train services could also be disrupted, the Met Office warned.
:: UK weather – the latest Sky News forecast
On Monday, conditions should improve in Wales, the West Midlands, the South West and North West.
But the East Midlands and Yorkshire and the Humber will see the yellow warnings continue.
The North East and eastern parts of England could also see lightning strikes.
Monday’s alert runs between 10am and 11pm.
It follows a summer of relentless sunshine and hosepipe bans.
Monday will see temperatures of between 19-22C (66-72F) in Wales, England and Northern Ireland – with 15-16C (59-61F) in Scotland.
Looking further into the week, Sky News weather presenter Jo Edwards said: “Tuesday will be a better day for central, southern and southeastern areas, where it will stay mostly dry with some good sunny spells.
“Northern and western parts of the country will see extensive cloud with patchy rain and drizzle for western Scotland and Northern Ireland.”