As Brits swelter in temperatures that rival Jamaica, the Met Office has issued an amber warning for some parts of the United Kingdom until Friday this week, and urged people to stay indoors.
"Where the thunderstorms occur torrential downpours are possible with as much as 30mm of rainfall in one hour and 60 mm in three hours".
The hottest July day on record saw the mercury hit 36.7C at Heathrow on July 1 2015.
"There is greater potential for both the July and the other (all-time) record to go tomorrow", said a Met Office spokesman noting that the highest temperatures are expected in the southeast - in Kent, Essex, Suffolk and Cambridgeshire.
"Will we beat the United Kingdom record temperature of 38.5C (101.3F) recorded at Faversham in Kent on 3 August 2003?"
Temperature records could be broken later this week as the 2018 heatwave continues.
Deputy chief executive of NHS Providers, Saffron Cordery, said: "These days there is no real let-up in the pressure on NHS services in summer in the way we used to see".
MET OFFICE AFFECTED AREA Thunderstorms are expected to batter parts of England this evening
"We have heard concerns about large numbers of people from care homes requiring treatment".
A high pollution warning was issued for London on Thursday, due to a mix of toxic air, extremely high temperatures, emissions from the continent and a lack of cloud.
London Fire Brigade's Commissioner Dany Cotton urged all borough councils to impose a temporary ban in the "tinder dry capital" as it was "only a matter of time" before someone was seriously hurt.
Stay out of the sun.
The extreme temperatures have also led to a temporary relaxing of the strict dress code for cricket spectators in the Pavilion at Lord's. The Heat Health Watch Service is created to help healthcare professionals manage through periods of extreme temperature.
"Southeastern parts of Britain will see the hottest conditions although temperatures will be well above average across the UK".
A parliamentary environment committee has warned heat-related deaths could treble and rise to 7000 a year by 2050 unless the government plans ahead with strategies to protect people, especially the elderly.
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