Temperatures are set to plunge again overnight after one of the coldest starts to December in more than 20 years has caused chaos across the UK.
Some 4,000 schools have been closed, and Edinburgh and Gatwick airports will be shut until at least Thursday.
Transport secretary Philip Hammond has called for an urgent review of how well the Highways Agency has coped.
The Met Office has issued heavy snow warnings for Scotland and north-east, eastern and south-east England.
Temperatures were at -16C (3F) in the Highlands on Wednesday, and the Forth Road Bridge was closed for the first time since it opened in 1964 but has now reopened.
In England, heavy snow warnings are in place for the North East, Yorkshire and Humber, East, and London and the South.
In Scotland the warnings apply to Grampian, Strathclyde, Central, Tayside and Fife, and South West, Lothian and Borders.
A Gatwick spokesman said: "Given the latest weather forecasts, Gatwick's runway will remain closed until at least 10am tomorrow [Thursday], which means no flights will depart or arrive during this time. There will also be significant disruption, delays and cancellations."
More than 250,000 Scottish children have had a day off - about 40% of all those of school age - many for the third consecutive day, with a third of councils closing all their schools.
Some local authorities have told parents that schools will remain closed for the rest of the week. In total, more than 1,500 of 2,722 schools were shut.
An avalanche warning was issued in the Cairngorms, near Aviemore, a popular location for skiers.
Police were advising people not to travel unless absolutely necessary - especially in the worst-hit areas of Scotland, Yorkshire, Derbyshire and south-east England.
Supt Chris Moon, of Surrey Police, said the county's conditions were the worst he had ever seen and were likely to deteriorate further.
He added: "I have put out several severe weather warnings in my career, but this one I really must stress."
Police in Kent advised freight traffic not to enter the county unless absolutely necessary as gale force winds are forecast. Some roads in the west and north of the county were impassable due to snow and ice.
Thousands of rail commuters faced severe disruption to their journeys in Scotland and northern England, particularly in the Sheffield area. London and the South East also suffered.
Southeastern trains, which runs out of London Victoria and Charing Cross, is operating an emergency timetable and services will finish early.
There were delays on the East Coast Main Line, with an hourly service between London and Edinburgh, and reduced services between London and both Leeds and Newcastle.
Half of Eurostar's services between London and Brussels on Thursday have been cancelled, as have seven out of 17 services each way between London and Paris.
National Rail Enquiries has set up a hotline for information about snow-related disruption on 08453 017 641, and also has updates at @nationalrailenq on Twitter.
About one third of all rail services were suffering delays or cancellations at Wednesday lunchtime.
Ashwin Kumar, of rail watchdog Passenger Focus, called on train companies to do all they could to make life easier for their customers.
He said: "We need better information systems so that we can cope when disruption does happen."
The Association of Train Operating Companies said rail operators and Network Rail were "doing all they can to keep trains moving and get people to where they need to be".
One of the worst affected areas of England is South Yorkshire, where snowfall of up to 30cm (12in) has brought parts of the county to a standstill.
About 300 schools were shut, bus services suspended in Sheffield, Rotherham, and Doncaster, and there were delays and cancellations on train services between Sheffield and Leeds, as well as flights being halted at Robin Hood Airport in Doncaster.
Hospitals in South Yorkshire asked patients not to attend unless urgent, and they asked off-duty medical staff to make their way into work if possible.
Sheffield City Council cancelled a full council meeting for the first time and the Halifax Courier newspaper did not make it out, also for the first time, because it was stuck at the printers in Sheffield.
The Lincolnshire village of Binbrook has been effectively cut off since Monday. Local shops have begun to run out of supplies.
About 100 motorists will spend the night at South Anston methodist chapel near Sheffield, after being stranded on the A57 since Tuesday evening.
Police say they are increasingly concerned about fell walker Gwenda Merriot, 60, from Wiltshire, who was last seen in Ambleside in the Lake District on Wednesday morning. Heavy snowfall was forecast for the area overnight.
Temperatures hit a low of -20C (-4F) in Altnaharra in the Highlands overnight on Tuesday.
BBC weather forecaster Tomasz Schafernaker said there was 1m (3ft) of snow across parts of north-east England.
More heavy snow was expected on Wednesday afternoon and evening across southern England from Hampshire to southern Essex and possibly into London.
There could be up to 30cm of snow in southern England by the end of Thursday. There will also be more snow showers in north-east England.
On Thursday night, temperatures of up to -25C (-13F) to -30C (-22F) in some parts of Scotland are forecast.
The disruption on Wednesday included:
In Sheffield, bride-to-be Tracy Gell had to call off her big day because it turned out to be too much of a white wedding.
She told the BBC: "I had a text from the photographer who couldn't get through, the wedding car couldn't get through, so I decided: I can't wear my dress without ruining it, so we'll postpone."
On Tuesday evening many drivers and rail passengers were stuck for hours as the weather worsened.
Tony Scott started his journey home from London to Tonbridge in Kent at 1730 GMT on Tuesday and had still not arrived home 10 hours later.
The AA had attended 11,300 breakdowns across the UK by 1500 GMT on Wednesday, with the busiest areas said to be London, especially the south of the capital, Yorkshire, Kent, north-east England and Aberdeen.
The motoring organisation said there had been a failure to deal with jams on motorways and major roads, and breakdown service Green Flag said local authorities had not spread enough grit on minor roads.
Mr Hammond said: "Decisions about closing the motorway would be decisions for the Highways Agency and enforcement of traffic regulations are of course, a matter for the police," he told the BBC.
"We will work together with all the agencies concerned to look at what has happened, to look at how best, and how most effectively, to tackle the problems that we have seen in order to get the motorways working, to get the railways working, and to get Britain back to normal as quickly as we possibly can."
Halfords said an additional 16,600 sledges were being shipped in to meet increased demands.