Sunday is set to be the hottest day so far this year and temperatures may hit 30C, the BBC weather centre predicts.
The previous high for 2010 was on 24 May - 28.8C at Heathrow.
The prediction comes as millions are set to watch England in the World Cup, tennis fans flock to Wimbledon and Glastonbury festival-goers bask in the sun.
"The heat just continues to build and build and Sunday could be its peak," BBC forecaster Dan Corbett said.
The hot weather has been caused by an area of low pressure to the north-west of the UK which is allowing warm air to travel in from southern Europe.
"There could be some cloud and showers across parts of north west Britain and even some thunder. The south-east of England up to Birmingham will see the highest temperatures," the BBC forecaster said.
William Hill, meanwhile, has set odds of 16/1 that temperatures will top 100F at Wimbledon and 5/1 that the temperature will break the 100F mark at any time this year.
At Wimbledon officials have taken measures to ensure spectators do not wilt in the heat at the All England Club.
Some 1,000 bottles of suncream and 8,615 punnets of strawberries are expected to be sold.
Those attending Glastonbury can swap wellies for flip-flops and there will be plenty of sunbathing opportunities between gigs, with thermometers topping 25C.
Bangers and bikinis
Medics at the festival have treated 1,185 people so far, mostly for heatstroke and other sun-related ailments.
Tesco said it expects to sell one million burgers, nine million fresh sausages and 700,000 bags of salad as people plan to hold parties to mark the weekend's sporting events.
Asda said it has sold 165,000 bikinis in recent days, up 47% on this time last year.
Meanwhile, the threat of a hosepipe ban is looming over residents of north-west England.
United Utilities, the water company that provides water to the region's seven million people, has applied for a drought permit.
Source : BBC News