The mercury hit 33.9C in Charlwood in Surrey on Thursday, the Met Office said - and it is set to get even hotter.
Forecasters say temperatures could soar to as high as 37C on Friday, triggering a thunderstorm warning into the afternoon.
With the potentially record-breaking high temperatures in the coming days, the Government is being warned it must do more to protect vulnerable people in extreme heat.
There is a chance the high temperatures could even beat the all-time UK record of 38.5C, with the hot weather set to continue into August.
A Met Office spokesman said while it will be a bit cooler and potentially unsettled over the weekend, the hot weather will return next week.
A mix of toxic air, extreme highs, emissions from the continent and a lack of cloud cover have caused a “high” air pollution alert to be issued for London.
Children, the elderly and people with lung or heart problems have been advised to reduce strenuous exercise and physical exertion.
Advice and warnings have been issued to those on the move in the heat.
Drivers have been urged not to throw rubbish from vehicles amid a surge in grass fires near busy roads.
There were 56 fires on grass verges next to England’s motorways and major A-roads in the 10 days to Wednesday, Highways England reported.
Passengers using cross-Channel Eurotunnel services face delays of up to five hours after air conditioning units failed on trains.
The English end of the Folkestone-Calais link was worst affected, with people forced to queue for up to 90 minutes just to check in and a further three-and-a-half hours for the next available slot, according to the operator.
Footage posted on social media showed long queues of cars, caravans and lorries.
Meanwhile, London Fire Brigade is calling for a temporary ban on barbecues in public parks after a record-breaking spate of grass fires in the tinder dry capital.
London Fire Commissioner Dany Cotton, who has written to all London councils suggesting the move, said: “I have attended a number of these large grass fires and it never ceases to shock me how many abandoned smouldering barbecues fire crews spot.
“In these arid conditions, barbecuing on dry grass is not just thoughtless, it’s reckless.”
Leicestershire County Council said that since the beginning of July, the council’s gritters have spread around 150 tonnes of granite over roads at risk of melting.
It is the first time for a number of years the highway teams have had to carry out dusting using modified gritters.
Elsewhere, Health Secretary Matt Hancock has been urged to set out what measures he is taking to make sure the NHS can cope with the heatwave.
Shadow health secretary Jon Ashworth said hospitals are facing year-round pressures, saying “the traditional lull in NHS activity during the quieter summer months is a thing of the past”.
Blood supplies have dropped, health officials have said, saying some people have been too dehydrated to donate while others have chosen to stay in the sun.
One nurse was admitted to A&E with dehydration after working three 12-hours shifts in a row during the heatwave, the Royal College of Nursing (RCN) said.
MPs have warned heat-related deaths will treble by mid-century unless the Government takes action to tackle the dangers of soaring summer temperatures.
Scientists have warned that climate change is making such heat extremes more likely, and the report comes as northern Europe swelters in a prolonged heatwave.
Wildfires in Greece have claimed the lives of at least 80 people, including Irishman Brian O’Callaghan-Westropp, who was there on honeymoon.
He and his new wife Zoe Holohan, who is in hospital after suffering burns to her head and hands, had flown to Greece on Saturday following their wedding in Co Meath two days earlier.