Christmas shoppers ditched their winter woollies and donned shorts and t-shirts on one of the warmest December days on record.
Scarves, hats and gloves were temporarily cast aside as temperatures hit 16.3C in the UK - higher than this year's nationwide July average.
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The highest the mercury has ever climbed is 18.3C, recorded in the Scottish highlands in 1948.
The Met Office said temperatures peaked at 16.3C at Achnagart in the Scottish Highlands this morning - more than double the normal average of 7.2C.
Experts attributed the warm spell to high pressure bringing settled weather to the east and colder conditions across parts of the USA and Canada strengthening the jet stream, pulling warmer air into UK.
The mild conditions are expected to continue for a few days, although temperatures were tipped to peak on Wednesday.
Among those making the most of the exceptional conditions was John Miskimmon, 34, who browsed festive food and gifts at Belfast's bustling Christmas market in just a t-shirt and trousers as temperatures reached 15.8C in Northern Ireland.
He said: "I am loving this weather. It's great to be able to walk about in a t-shirt and not have to worry about a coat.
"I might run back and put my shorts on it's so warm."
Meanwhile, retired health service worker John Gorman switched his indoor winter training regime for a run through Ormeau Park in south Belfast.
The 67-year-old said: "I normally train indoors but because it is so nice I thought I'd take it outside and go for a run through the park. It's unbelievably mild.
In Cambridge, tourists in t-shirts enjoyed punting as temperatures climbed into double figures.
Punting companies are often forced to close in December as the River Cam ices over, but this year tours are still in full swing amid the unseasonable weather.
Temperatures reached double figures across the country, with highs of 13C in the north of England, 15C in the south of England and 16C in Scotland.
The record for England is 17.7C, measured in Devon in 1985 and Staffordshire in 1994.
The record for Wales is 18C, reached in 1972 in Gwynedd.
Today brought windy weather in the north-west England and a band of rain moving south-east as the day goes on.
Tonight was also forecast to be much milder than it has been recently, with temperatures remaining in double figures.
But forecasters warned people to make the most of the mild weather and as cold temperatures are set to return next week.
Simon Keeling, of WeatherOnline, said: "An unsettled and very mild few days lie ahead as the southwesterly airflow maintains its grip across the UK.
"There will be outbreaks of rain and drizzle, mostly in the west.
"Hints are that as we head into the early to middle stages of next week higher pressure may start to build back from the east bringing some drier conditions and cooler temperatures."
Tomorrow will bring heavy rain, which will gradually clear in Wales and central and northern England during the morning. It will be breezy but drier and brighter in the north.
Temperatures will drop slightly on Friday and Saturday, with spells of rain across much of the country on Saturday. Sunday will be the best day of the weekend with more settled weather.
Last year December broke records for both rainfall and temperatures.
The UK's mean temperature for December 2015 was 7.9C - 4.1C above the long-term average and 1C more than 1934's previous record of 6.9C.
It was also the wettest December on record and the wettest calendar month since records began in 1910.