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The UK's all-time temperature record could be broken by Friday as the hot and dry summer continues.
Highs of up to 37C (99F) are forecast in southern, eastern and central England, but the Met Office says "if conditions all come together", the record of 38.5C (101F) could be beaten.
That was set in Kent in August 2003.
Fire crews are urging people enjoying barbecues and other outdoor activities to avoid "careless and reckless" behaviour that could start a blaze.
In London, firefighters have already attended six times as many large grass fires this year as in the whole of 2017.
Tom George, deputy commissioner and director of operations for London Fire Brigade, said the force was "fed up of politely asking people to take care".
Across the country, other forces are issuing similar warnings about the risks posed by cigarettes, glass bottles, barbecues and camp fires when the ground is tinder dry.
Sweltering Londoners forced to take the Underground have found scant comfort in the news from Transport for London that air conditioning is coming to the Central Line - in 2030.
Paul Hedge tweeted sarcastically: "Not too bad then! Only another 11 and a half years to go."
Some overground trains are also being forced to operate at reduced speeds because of the risk of tracks buckling.
Chiltern Railways trains between London and Oxford are being rescheduled between noon and 18:00 BST each day until Friday.
Hospitals, too, have been affected by the heat.
Kim Sun-Lee, national officer at the Royal College of Nursing, told the BBC: "We know wards are reaching over 30C, nurses are feeling sick, dizzy, having problems with their concentration.
"And that then does have an impact on how they care for their patients, making decisions around drugs calculations and just generally doing a physical occupation."
Sunscreen warning
Extreme temperatures on Thursday and early Friday are likely to lead to some respite, however, with thunderstorms expected to follow in central and eastern England.
The Met Office warned those storms could bring "torrential downpours" as well as "large hail and strong, gusty winds".
While it remains hot, members of the public have been urged to take precautions amid research suggesting people are getting less than half the sun protection they expect from suntan lotions.
The problem, King's College London scientists say, is a result of the way people apply lotion to their skin.
Meanwhile, in Derbyshire, the heatwave has exposed the remains of an ornate 17th Century garden design at Chatsworth House.
The patterns of where flowerbeds and paths once were can be seen under the South Lawn's scorched grass.
Head of gardens Steve Porter said he had known the designs that date back to 1699 were hidden underneath the lawn.