A heatwave warning has been issued for England and Wales, with temperatures in some areas potentially rising as high as 32C (90F) in the coming days.
The Met Office says London, the east and south-east of England will be the hottest areas, both by day and night.
Warm and humid weather is also forecast throughout the rest of the UK, with a risk of thundery showers.
The Department of Health has asked people to check on elderly friends and family who may suffer in the heat.
NHS staff have also been warned to prepare for a surge of elderly and ill patients suffering from the heat.
Hospitals could be asked to set up emergency "cool rooms" and hand out cold drinks as part of a string of measures set down in the Government's official heatwave plan.
The Met Office said Monday has been the hottest day of the year so far, with Charlwood, in Surrey, recording a temperature of 28.4C.