Thunderstorm warnings have been issued for large parts of the country, lasting from lunchtime until Monday night.
The Met Office says thunder and heavy downpours could cause flooding and travel problems.
Southeast England – including Kent, Sussex, Surrey, Hampshire and south London – could be affected from noon, says the forecaster.
Other areas affected are mid-Wales and most of northern England, stretching up past Glasgow into southwest Scotland.
That warning is in force from 1pm – with both alerts ending at 9pm.
Pic: Met Office
Pic: Met Office
“Spray and sudden flooding could lead to difficult driving conditions and some road closures,” says the Met Office, while there is also a small chance that roads and properties could be flooded.
Bank Holiday Monday has brought a grey and cloudy start for many, with forecasts ranging from 12C (53F) in the Hebrides to 18C (64F) in northern England at 1pm.
Get the latest Sky News forecast for where you live
However, it will become warmer, brighter and drier from Tuesday with high pressure forecast for much of the UK – and it should stay that way for next weekend.
Read more from Sky News:
Woman arrested after baby has cardiac arrest at Legoland
Sunak admits Tories may not win general election
London is expected to have 23C (73F) with sunny spells next Saturday and Sunday, according to the Met Office.
Highs of 20C (68F) are forecast in Edinburgh, while Cardiff is set for sunshine and 19C (66F), and Belfast likely to struggle with cloud but still feeling warm with a maximum of 20C.