Today is officially the hottest day of the year so far, with temperatures reaching 37.8C (100.04F) in Greater London.
The Met Office tweeted that Friday has been the "hottest day of the year by some way".
Met Office spokeswoman Nicola Maxey confirmed the highest temperature was recorded in Heathrow, Greater London, on Friday afternoon.
She told Sky News: "Today is the hottest day of the year so far as 37.8C (100F) was reached at 2.41pm at Heathrow today.
"37.3C (99.1F) was reached at Kew Gardens at 2.55pm.
"Before today, only three days on record have seen temperatures that exceed 37C."
Forecaster Simon Partridge added: "This not only makes it the warmest day of the year, it also makes it the third hottest UK day on record."
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The current temperature record was registered at 38.7C (101.7F) at Cambridge University Botanic Gardens last year, while the second hottest day was 10 August 2003, when the mercury rose to 38.5C (101.3F) in Faversham, Kent.
The Met Office earlier warned that climate change was exerting an "increasing impact" on the UK, with data showing an "undeniable warming trend".
The soaring temperatures seen on Friday are expected to be short-lived.
The Met Office issued a thunderstorm warning for Friday afternoon, with a yellow warning in place from 4pm until midnight across large parts of East and South East of England where the weather will be at its hottest.
Gusty winds, hail and "frequent lightning strikes" can be expected, which could cause slight damage to buildings.
The Met Office said the storms will be caused by hot air moving in from mainland Europe, some parts of which will be cooler than the UK.