The weekend heatwave culminated in the hottest day of the year with temperatures hitting the 30C (86F) mark.
Forecasters said the south-east of England would remain warmer than average for the rest of the week while showers and cooler weather were on the way for the rest of the UK.
The mercury reached a high of 30.9C (87.6F) at Gravesend in Kent, making it the hottest day of the year.
London and the south-east of England saw the warmest weather, on a par with Mediterranean holiday resorts.
St James's Park in central London reached 30.1C (86.2F) and Heathrow Airport had a high of 30.6C (87.1F).
Sunshine dominated across the UK but a westerly wind kept temperatures down to 19C or 20C in western parts of England and Wales while Scotland and Northern Ireland experienced showers, forecasters said.
Before today, the hottest day of the year was May 24 when the temperature reached 28.8C (84F) at Heathrow Airport and the Olympic Park in east London.
The sweltering conditions prompted bookmakers to cut the odds of a record-breaking summer as thousands flocked to beaches or soaked up the rays with barbecues and parties in the garden to watch the World Cup.
Music fans enjoyed the "hottest ever" Glastonbury festival - ditching their wellies for the first time in years.
Weather forecaster Gareth Harvey, from MeteoGroup, the weather division of the Press Association, said temperatures would drop slightly after Sunday with rain predicted. He said: "It looks as though it's going to stay very warm into next week, especially across the south-eastern quarter of the UK."
