Temperatures have hit 18.3C (64.9F) as the country basks in the warmest day of the year so far.
The temperature, which also marks the warmest day of the winter so far, was recorded at Kew Gardens, west London and Northolt, north-west London.
Heathrow Airport saw highs of 18.1C (64.6F), while 17.5C (63.5F) was recorded in Marham, Norfolk.
Nicola Maxey, a Met Office spokeswoman, said the balmy temperatures were caused by tropical Atlantic air sweeping across the country.
"From tomorrow temperatures start dropping - much more near the average temperature for the time of year," she said.
She added that by the end of the week temperatures could be closer to single figures, with London expected to reach 10C (50F).
Monday's high edges towards the highest February temperature since records began - 19.7C (67.5F) at Greenwich Observatory in south-east London, on February 13 1998.