The UK could have had its hottest ever day in December after the Met Office said it has provisionally recorded a new maximum temperature of 18.7C (65.66F).

Forecasters took the measurement at Achfary in the Highlands on Saturday, which would be the hottest day of the month on record in Britain if validated.

It would beat the current top December temperature of 18.3C (64.94F), which was taken at Achnashellach in the Highlands in 1948.

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A new provisional maximum temperature of 18.7 °C was recorded at Achfary in Scotland on the 28th December 2019



It will be quality controlled & if validated would be the highest temperature officially recorded in the UK during December pic.twitter.com/FXVMXvcR5I — Met Office (@metoffice) December 31, 2019

At 3am on Sunday the Met Office recorded a temperature of 16.8C at Cassley in Sutherland, which was the highest on record in Scotland for a 29, 30 or 31 December.

The unseasonal Scottish warmth has been put down to a meteorological phenomenon known as the Foehn Effect. This is a change from wet and cold conditions on one side of a mountain to warmer and drier conditions on the other side. Dry air heats more quickly so as it descends it warms up much faster than on the cold side.

The most notable Foehn events tend to occur across the Highlands, where the moist prevailing westerly winds encounter high ground along Scotland’s west coast.

On Saturday, the northern part of Scotland was experiencing southerly winds.

The lowest temperature recorded on the same day was 1.7C (35.06F) in Writtle, Essex.

If the temperature is confirmed it will be the second time a temperature record for the Highlands has been broken this month.

A record temperature of 16.8C has already been verified for Cassley in Sutherland, which was recorded at 3am on Sunday 29 December.

Northern Wales and north-east England were also unseasonably warm on Saturday night. Temperatures of 13.3C were recorded in Chillingham Barns, Northumberland and 11.5C in Rhyl, north Wales.

Alex Burkill, a meteorologist for the Met Office, told the BBC: “Getting temperatures of 16 or 17 degrees in December isn’t all that unusual but it’s remarkable that this was during the night.”