The weather could soon be as gloomy as the Chancellor's economic forecasts, with rain predicted to replace the sunshine by the end of the week.
Temperatures reached 21C (69.8F) in central London, making it hotter than Paris and Rome.
Typical temperatures in London at this time of year are around 14C (57.2F).
Paul Mott, a forecaster with MeteoGroup, the weather division of the Press Association, said the sunny weather was not reserved for the south-east with much of the country enjoying higher than usual temperatures.
He said: "South east England was certainly warm during the day but it was warm in much of the rest of England, Wales and eastern Scotland with temperatures between 16C (61F) and 18C (64F).
"It was somewhat cooler over Wales and Northern Ireland with temperatures between 12C (54F) and 15C (59F)."
But he said the weather would take a turn for the worse towards the end of the week with rain replacing sunshine over the weekend.
He said: "I think eastern England will still be reasonably warm but it will be cooler in the west, between 12C (54F) and 15C (59F).
"There is potential for heavy rain over Northern Ireland, western Wales and the south west of England for Saturday and right through the weekend."
