Thursday is expected to be "exceptionally" mild, with temperatures up to 10 degrees Celsius above average for the time of year, forecasters said.
The mercury is expected to reach 17C (63F) in eastern parts of the UK - against an average of 7C (45F).
Chris Burton, a forecaster with MeteoGroup, the weather division of the Press Association, said: "It will be exceptionally mild for this time of year."
The day is unlikely to set any records, though - the warmest February day in the UK was in Cambridge in 1891, with a reading of 19.4C (66.9F).
There have also been readings of 18C (64F) in 2007 and 17C in 1998.
Mr Burton said: "The warmest temperatures tomorrow are expected to be in East Anglia, Lincolnshire, Yorkshire and eastern parts of Scotland. Most of the UK will have above average temperatures, as in the last few days. Temperatures up to mid teens will be quite common, and when the sun breaks through, in eastern parts, they will go that bit higher."
Conditions are expected to turn a little less mild through the weekend, but will remain a little above average for the time of year.
Rain is expected to be mainly in the north and west, with no significant falls expected in the South East, where there is a shortage of water.