The Met Office has issued a yellow alert for rain, ensuring the public to "be alert" of potential torrential downpours today.
A Met Office forecaster said: "Isolated thunderstorms with very heavy rain, hail and gusty winds are possible later this afternoon and early evening.
"The public should be aware of the risk of very localised surface water flooding, particularly if the thunderstorms affect urban areas".
"These could produce 30 mm of rain in less than an hour, large hail and also squally winds".
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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.