The sun is set to fend off a chilly start before temperatures reach as high as 21C in south London and Surrey today.
Forecasters warned temperatures would "rapidly rise" as the sun breaks through any early morning cloud to continue the sunny spell.
Although the highest temperatures were expected across Oxfordshire, Berkshire and Hampshire, the Met Office said temperatures could still reach 21C in London and Surrey.
However, the clear skies meant temperatures would drop rapidly this evening, a forecaster said.
"Clear skies and very light winds will allow the temperature to quickly fall this evening and overnight, with a slight ground frost developing in rural locations," he warned.
Temperatures could drop as low as 2C.
But things were set to warm up again on Wednesday, with temperatures hitting 23C.
"Becoming exceptionally warm, after a chilly start to the day, due to the unbroken sunshine," the forecaster said.
"However, light sea-breezes are likely to develop through the afternoon, keeping coastal areas slightly cooler."
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