The UK is facing one of the coldest winters in 100 years with conditions causing "widespread, persistent and severe" problems this week, forecasters warned.
With up to 4ins of snow due to fall in parts of the country today, heavier deluges are predicted to hit many others later this week causing transport chaos on roads, trains and at airports.
MeteoGroup, the weather division of the Press Association, said up to 15cm of snow could end up lying over "wide areas" by Friday, with the south experiencing "significant or even major difficulties to infrastructures, particularly transport".
Temperatures fell as low as minus 10C (14F) across southern England overnight, with no end in sight to the freezing weather.
Stephen Davenport, senior meteorologist at MeteoGroup, said: "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.
"Should conditions continue in a similar vein then by March we might just be looking back at one of the coldest winters of the last 100 years."
He added: "There are more immediate concerns in the shape of a very snowy spell this week.
"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.
"By Friday or even sooner there will be several centimetres of snow lying over wide areas, to depths of five to 10, or even 15, centimetres. In a few ill-favoured areas the persistence and heaviness of the snow could bring accumulations well in excess of 20 centimetres."
Today a band of snow will move down from Scotland bringing showers to northern England, northern Wales and possibly Northern Ireland.
By mid-afternoon it will have reached the south east and by the evening there could be disruptive snowfall in the south, the south Midlands and south Wales.
Temperatures will barely top freezing during the day, with London set to be the warmest at 3C (37F). Manchester could possibly touch 1C (34F), Glasgow will hover around 0C (32F) and Cardiff 2C (36C).
Commuters suffered more disruption yesterday as they returned to work after the Christmas break, with thousands of school children enjoying an extra day off due to the weather.
Yesterday the AA was experiencing its busiest ever day, with the organisation on schedule to attend more than 25,000 breakdowns over the 24 hours.
The AA also warned that the roads were likely to be even busier today when worsening weather conditions combined with the return to school in many areas.
Grit stocks in England are holding up according to the Highways Agency, but Fife council in Scotland had to have several hundred tonnes of salt and grit delivered by the Scottish government after supplies ran low.
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A Scottish Government spokesman said Fife Council received 250 tonnes of salt and grit yesterday following a delivery of 50 tonnes the day before.
Highland Council also said it had used 30,000 tonnes of salt over the past three weeks - more than the total it used during 2006/7.
Yesterday, over-running rail engineering works caused 60-minute rush-hour delays to trains in and out of London's Liverpool Street station.
And on London Underground there were part-suspensions on the District and Hammersmith and City lines due to signal failures.
Even the fountains in Trafalgar Square iced over in the sub-zero temperatures.
Elsewhere Merseyrail services were delayed by up to 30 minutes and rail passengers were hit by poor weather in Scotland which led to delays between Glasgow and Edinburgh.
Buses replaced trains between Ormskirk and Preston and between Boston and Skegness in Lincolnshire, while a signalling problem near Dagenham Dock in Essex led to delays on services to and from Fenchurch Street station in London.
Bookmaker Paddy Power cut the odds on this being the coldest January on record from 5/1 to 7/4.
Ten train services between London and Leeds on the East Coast Main Line were cancelled today.
Several London to Glasgow services on the line were also stopping at Edinburgh, with a number of others affected.
The line operator said it would be operating a revised timetable today due to the adverse weather conditions.
Budget airline easyJet cancelled a number of flights on its European network, including two between Luton and Milan.
The Highways Agency also said the A66 in Cumbria was closed both ways between the junctions with A1 and A685. The closure is expected to last for several hours.
Manchester Airport said it had closed due to heavy snow, but was due to open around 9am.
Passengers are being urged to check their flight status with the airline.
Glasgow Airport said it was open, but heavy snow and icy conditions across the UK would result in delays and cancellations.
Travellers were also being advised to check with their airline.
Tonight (Tuesday night):
Snow showers expected across Northern Ireland with widespread sharp frost tonight. Many areas will see outbreaks of heavy snow continue into the night, the heaviest of which can be expected in southern England, with the ongoing risk of snow in many areas until the early hours of the morning. It will be drier in Scotland with patches of mist and fog forming but snow will continue in the far north and the north-east.
Mostly dry and bright across Northern Ireland. Snow showers across northernmost Scotland and parts of the north east, particularly across coastal counties. Elsewhere in Scotland it will be largely dry with plenty of bright spells. It will be rather cloudy with a scattering of wintry showers across northern England. Elsewhere in England will see outbreaks of snow, this often heavy with accumulations expected. Scattered wintry showers in Wales, these mostly confined to eastern areas with just the odd coastal shower. Any outbreaks of snow across the UK will start to clear south-east through the evening.
UK Outlook (Thursday and Friday):
Another cold day on Thursday. Wintry showers, these falling as snow at times across N England and parts of the Midlands. Coastal parts of East Anglia, the south-east of England and north Wales will see isolated showers. Elsewhere it will be mostly dry and bright with mist and fog patches across Scotland. On Friday, eastern areas of Scotland and England will see wintry showers, these falling as rain and sleet along coastal areas. Elsewhere, it is expected to be largely dry and bright, however there is the risk of the odd shower in northern regions of Northern Ireland.