Meteorologists have recorded the hottest day in April since 1949, with temperatures reaching 28.8C (83.8F) in the capital.
The Met Office recorded the highest temperature in Northolt, west London, on Thursday afternoon.
Thursday's temperatures follow a high of 25C (77F) on Wednesday afternoon in Gravesend, Kent, and St James's Park in London.
The UK is currently seeing temperatures hotter than Athens, where it is 25C (77F), and in Madrid, where it is 22.2C (77F).
British temperatures are also higher than Rome, where it is 25C (77F), and in Rio de Janeiro, where it is 26C (79F).
The scorching heat is a stark contrast to the average maximum temperature for April, which is a mere 11.4C (52.5F).
The hottest April day recorded in the UK was in London's Camden Square in 1949, which saw temperatures of 29.4C (84.9F).
The Met Office said warm air off the coast of Portugal had been dragged up towards the UK by an area of low pressure over the Atlantic and high pressure over western Europe.
Spokesman Oli Claydon added: "After quite a dull and chilly start to spring this is the first bout of real warm sunshine that we've seen.
"The sun at this time of year is nearly equally as strong as it can be in the summer so people just need to be cautious of that and make sure that they do look after themselves."
People planning to run Sunday's London Marathon in fancy dress have been urged to reconsider as the hot weather continues into the weekend.
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However, temperatures are set to fall across the UK from the beginning of next week, with some showery outbreaks predicted.
The Met Office's forecast for Sunday into Monday is: "Thundery showers in the north and west becoming more widespread. Unsettled, cooler and breezy on Monday."
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