Today (Sunday) has provisionally been recorded as the warmest Christmas Eve since 1997, according to the Met Office.
The weather agency said temperatures reached 15.3C at Heathrow, west London.
The warmest Christmas Eve on record was in 1931, when 15.5C was recorded in Aberdeen and Banff in Scotland.
Meanwhile, Christmas Day is expected to be the hottest in years as some parts of the UK are predicted to hit 14C.
Most of the country will remain mild and cloudy with drizzle at times ruling out a white Christmas, apart from the mountains in northern Scotland which could see snow.
Temperatures are unlikely to exceed the warmest Christmas Day on record when the temperature reached 15.6C in 1920.
However, Met Office forecaster Dan Stroud said this Christmas could see the “warmest Christmas day since 2016” as the UK hit 15.1C.
Met Office forecaster Liam Eslick said: “It’s been an exceptionally mild couple of days across all of the UK, temperatures have been well above average for the time of year, the maximum for December being 7C.
“Temperatures today have reached 15C in quite a few locations across the UK, the highest being 15.3C in two locations – one in Heathrow and one in Cippenham, Berkshire.”
The UK is drawing in warmer air from the south contributing to this unusually mild weather.
Two separate yellow wind warnings were issued by the Met Office in Scotland and northern and central areas of England for Christmas Eve, saying people should expect travel disruption, damage to buildings and power cuts, while the rest of the country may see showers.
In Wales persistent and heavy rain could cause disruption to transport and power supplies with the possibility of flooding.
And further yellow weather warnings have been issued by the Met Office for later in the week, including parts of Scotland which could face heavy rain and snow.