Forecasters have warned of a "reversal of fortunes", with scorching April temperatures set to fall by as much as 10C on Thursday.
A high of 25C was recorded in London's St James's Park on Wednesday as Britain enjoyed its hottest day of the year so far.
But tonight a spokesman for the Met Office said: "Areas that were cooler today will be a little bit warmer tomorrow. But in the South East, where we saw highs of 25C (77F), it will be up to 10 degrees cooler.
"A real reversal of fortunes."
At its peak today, parts of London were hotter than Athens, Rome, Nice and Ibiza.
The unseasonable highs benefited those able to soak up the sun in London's parks, while others endured uncomfortable journeys back from work after lines were hit by a series of problems.
The highest April temperature on record was 29.4C (85F) on 16 April 1949.
Additional reporting by PA
