A "dramatic" shift in the UK weather pattern this winter has brought an unusually prolonged freezing spell, according to forecasters.
Britain's prevailing wind generally brings weather from the west, but for the last few weeks the wind has blown from the north and east.
This has led to snowy conditions and sub-zero temperatures from the Arctic, Scandinavia and Siberia, which are predicted to continue for at least a week.
Brendan Jones, a forecaster with MeteoGroup, the weather division of the Press Association, said: "It has been one of the lengthiest cold spells we've had for quite a while, since 1995-96.
"It's purely because of the wind direction across the UK.
"The prevailing wind that our weather comes from is usually from the south-west and west and that means the weather usually comes from the Atlantic.
"We do get cold spells every winter but, because the weather tends to come from the west, winters are usually mild and wet with occasional cold spells.
"For the last three weeks, since mid-December, winds have come in from the north and east and everywhere in between.
"It has been the complete opposite of what usually happens.
"The wind has either been coming from the Arctic or from Scandinavia and parts of Siberia - all the cold places.
"Our source of warmer and milder weather has been completely cut off."
Mr Jones said the flow of air was going from east to west across the Atlantic, which meant parts of eastern Canada were "abnormally warm" at the moment.
"It's a big dramatic shift in the global weather patterns," he added.
MeteoGroup senior meteorologist Stephen Davenport said the very cold weather showed "no sign of relenting" for at least a week to 10 days.
"This is stretching the limits of short- to medium-term forecasting but so entrenched is this cold weather pattern that it seems only a major upheaval in the atmosphere will bring a return to something milder," he said.
"Should conditions continue in a similar vein then by March we might - might - just be looking back at one of the coldest winters of the last 100 years."
More freezing temperatures and snow are forecast to cause problems over the coming days, both in the north and south of Britain.
Mr Davenport said: "Heavy snow showers or more widespread snow will continue to move across northern and eastern areas of Britain, bringing further accumulations of several centimetres in places and causing notable disruptions to transportation.
"And the highly populated south is likely to see snow that will at times be widespread, persistent and severe enough to bring significant or even major difficulties to infrastructures, particularly transport."
Snow of 2in-4in (5cm-10cm) depths will be lying on the ground over "wide areas" by Friday at the latest, according to Mr Davenport.
In a few places, there could be 6in or 8in (15cm or 20cm) of snow, he added.
Forecasters said temperatures will stay below freezing at night for the next week, with daytime temperatures "struggling" to get above freezing across most of the UK.