Sunday has been declared the warmest Christmas Eve in the UK since 1997.
Temperatures hit 15.3C in Heathrow, west London, and Cippenham in Slough, the Met Office said.
There had been speculation it could be the warmest 24 December ever but that record remains consigned to 1931 - when 15.5C was recorded in Aberdeen and Banff in Scotland.
But the unseasonably mild weather lays the groundwork for what could be the hottest Christmas Day since 2016.
"The temperatures will peak today," Met Office forecaster Dan Stroud told the PA news agency. "There is a slight downward trend in temperatures for Christmas Day, but we're still expecting them to be comfortably above average."
He added: "We're looking at 13 and 14C tomorrow, we're probably looking at the warmest Christmas Day since 2016, when we hit 15.1C."
The warmest 25 December on record was 15.6C in 1920 - the temperature was recorded in Devon. The record for the warmest Christmas Day in Scotland is 15.1C, which was reached in Dyce in 2011 as well as in Urquhart, Ross and Cromarty, in 2016.
But while Christmas Eve was mild for many, forecasters said there had also been strong winds in some areas, with gusts of up to 56mph hitting parts of the Isle of Wight and Northern Ireland.
A yellow weather warning for wind stretching down eastern England from the Scottish border to the Midlands was put in place until 22:00 GMT.
The Met Office has said people in those areas should expect travel disruption, damage to buildings and power cuts.
Christmas Day is set to be damp for much of the UK, with heavy rain possible in Wales.
Most parts of Scotland will a see rain showers and relatively warm highs of 8C or 9C.
However, forecasters say a white Christmas is still likely in parts of northern Scotland.
They say snow could fall on higher ground in areas such as Caithness and Sutherland.
For Christmas Day to be classed as an "official" white Christmas, just one snowflake has to be observed within the 24 hour period.
