Freezing temperatures are forecast to continue for days, with Boxing Day blizzards likely in the West Country, Wales and Scotland.
Rail companies faced criticism for not running enough trains with enough coaches to accommodate customers on a day when millions of them were trying to travel with bulky luggage to reach their families.
Some operators cancelled a quarter of their train services yesterday, and some services will close completely over Christmas, with no trains running from Essex and East Anglia into London. Trips from London to Bristol and Wales will also take longer because of diversions around Reading, and engineering work is planned on 15 out of 17 lines between now and 3 January. The weather has meant disruption is still being caused to services run by East Coast, First Great Western, ScotRail and First Capital Connect.
Anthony Smith, chief executive of the pressure group Passenger Focus, said: "The message from passengers is clear – treat us like human beings and give us useful, timely information, early notice of delays, and more help from railway staff when delays happen."
The Association of Train Operating Companies admitted yesterday that trains were "over-crowded". Its spokesman Edward Welsh said that up to quarter of services had been cancelled in order to improve the lines' efficiency – reasoning he acknowledged sounded counter-intuitive.
"What the industry has been trying to do is concentrate on people getting through," he said. "That has impacted on punctuality and we apologise for that.
"We are trying to ensure that services are reliable so sometimes we have to reduce the number of services to increase the reliability of those services.
"Unfortunately it means some trains are overcrowded and it does mean cancellations, and we apologise for that, but trains have been getting through."
Heathrow operated almost a full schedule of 1,200 flights yesterday, but its operator BAA, which has faced criticism for the airport's lack of preparation for dealing with the snow, said it could not guarantee it would be able to get travellers to their destinations before Christmas. The backlog of stranded passengers stood at 600,000.
An inquiry into Heathrow's handling of the extreme weather conditions has been launched after the mass cancellation of flights. A panel of experts from airports and airlines around the world will scrutinise the "planning, execution and recovery" from the heavy snowfall which left hundreds of thousands of passengers stranded just days before Christmas.
Announcing the inquiry, BAA chief executive Colin Matthews said the panel would make recommendations to the airport owner, which has also approved an extra £10m for winter equipment: "The inquiry will forensically examine what went wrong at Heathrow, and look fundamentally at our ability to prepare and respond more effectively to periods of bad weather at an airport operating at its maximum capacity.
The disruption has not been not restricted to rail and air services, with the AA warning that icy roads and poor travelling conditions would see journey times double.
The association's crews were called out to twice as many breakdowns as normal, with 13,000 reported across the country by 4.30pm yesterday and the total expected to reach 19,000 by the end of the day.
Darron Burness, head of AA special operations, said: "Daytime temperatures haven't risen above freezing in most areas today, so ice will remain the biggest hazard for those travelling through the Christmas weekend."
Up to 75 councils out of a total of 205 are still waiting for orders of salt after a government report suggested the country may run out this winter if cold weather continued.
The majority of the UK should stay dry but cold today, with slight snow showers in northern and eastern parts of the country.
People trying to contact friends and family around the world have been affected by an outage to the free internet phone service Skype. The problems across its network, which can handle 25 million calls simultaneously, started at about 8.30pm on Wednesday night and continued until yesterday evening.
The disruption was caused by a software issue on critical parts of its network, the company said yesterday. A spokesperson for the Luxembourg-based firm said: "We are working extremely hard to ensure that everyone trying to sign in to Skype can access their accounts and make calls. We've identified a bug in the Skype software which was the root cause of the downtime."