Midwinter misery continued for travellers today, with road and rail journeys disrupted and delays and cancellations continuing at the UK's biggest airport.
As passengers struggled to get a flight at Heathrow, Transport Secretary Philip Hammond promised an inquiry into how services had virtually ground to a halt at the west London airport.
Commuters contended with a bitterly cold morning and many domestic rail services were disrupted, with some services cancelled, particularly in Scotland, and others delayed.
There were long queues for Eurostar Channel Tunnel trains at St Pancras station in London, while a number of accidents led to delays on some key road routes.
Some flights were possible in and out of Heathrow today and Mr Hammond said that night-flight restrictions would be lifted at the airport for the next four days.
But as Heathrow passengers complained about being treated like "complete idiots", Mr Hammond said the airport was likely to be operating at reduced capacity over the next few days.
Heathrow operator BAA admitted there had been problems and the company was "extremely sorry for those".
London mayor Boris Johnson was among those critical of the situation at Heathrow, while shadow transport secretary Maria Eagle said she would have expected Mr Hammond to lead "a much better, more co-ordinated response" to the bad weather.
Mr Hammond, who was making a statement to the House of Commons later today, said the railways were "broadly operating" and main roads were open and usable.
He said there had been "severe problems" at Heathrow where British Airways was forced to cancel all its short-haul flights scheduled after noon today.
David Cameron's spokesman said the Prime Minister thought Mr Hammond had handled the disruption to transport effectively.
American Suzie Devoe, 20, a Bristol University politics student, was one of many who had spent two nights sleeping on the floor at Heathrow.
Trying to get back to Washington DC to spend Christmas with her family, she said: "I just want to get home, I want to be with my family. But I'm being held in a horrible limbo. I can't relax."
She said she was booked on a flight on Saturday which was cancelled and she was unable to rebook because there was nobody at the BA Airways desk and no answer over the phone.
Her parents were eventually able to rearrange a flight for this morning but that was also cancelled.
At King's Cross station in London, dense crowds packed the waiting area as trains were cancelled or delayed.
Queues for the platforms stretched back to the doors as passengers waited, suitcases in hand, to leave the capital.
East Coast Main Line services were among those affected by the bad weather. In southern England there were delays and cancellations for commuters coming into London from south west England, from Bedfordshire and from East Anglia.
Eurostar services were delayed by speed restrictions in England and France. The company said it was not selling any more tickets up to and including Christmas Eve and that people without tickets should not come to St Pancras.
On the roads there were problems on the A38 in Devon, while the A380 route from Newton Abbot to Exeter was described as "complete chaos".
National Grid said it was forecasting a record demand for gas today at 465.8 million cubic metres (mcm).
The highest demand so far this winter has been 456.6 mcm on December 2, it said, with the current all-time record at 465.5 mcm set on January 8 this year.
Both the AA and the RAC predicted they would have their busiest days ever for breakdown calls. Both motoring organisations reckoned they could get as many as 30,000 calls today - well above the number for a usual Monday.
In parts of Gloucestershire 12in of snow fell and average temperatures dropped to around -10C, with some areas in the county dropping to as low as -17C, the county council said.
This has led to widespread ice forming on the roads with particular problems in urban areas of Cheltenham and Gloucester.
Councillor Stan Waddington, cabinet member for environment, said: "I completely understand people's frustrations, but when we're seeing temperatures as low as -17C no amount of gritting will make any difference.
"Unfortunately, grit does not work when it's this cold and there is simply nothing we can do about that.
"We will continue to do all we can to keep the roads moving, but I am afraid that there are no easy solutions here.
"I would advise people to make essential journeys only and to take extreme care if they have to go out in the car and check the forecast before leaving."
Inspector Keith Fagan, of Gloucestershire Police, said: "Due to the current adverse weather conditions we are continuing to advise drivers not to make their journey unless it is absolutely necessary."
Great Western Ambulance Service, which covers Wiltshire, Gloucestershire and the former Avon, said that despite the weather it continued to deliver an emergency service.
Associate director Andy Hollingshead said: "We received more than 2,000 calls in our control room over the weekend but we are continuing to reach everyone who dialled 999 with a medical emergency - although inevitably it is taking us longer to get to patients given the hazardous road conditions."
While weather-related incidents account for some of the increase in activity, many of the 999 calls are for serious clinical conditions made worse by extreme cold, such as breathing problems and chest pains.
Mr Hollingshead added: "People suffering those life-threatening conditions should continue to dial 999.
"However, for less serious conditions, we would urge people to think if they could receive the help they need elsewhere.
"This is probably better for them and helps us to reach those most in need of our clinical care."
Meanwhile all libraries in Bath and north-east Somerset closed early at 2pm.
At Bristol International Airport the snow has been cleared from the runway and taxiways and flights are currently operating.
A spokesman said: "However, delays may be expected throughout the day and passengers are advised to contact their airline to check the status of their flight before travelling to the airport.
"Sufficient time should also be allowed for the journey to the airport."
Shortly after 2pm BAA said it was snowing again at Heathrow and it was unable to accept any more passengers in the "extremely congested" Terminals 1 and 3.
A spokesman added: "We are therefore asking passengers not to travel to Heathrow Terminals 1 and 3 until further notice. We will review this at 4pm."
Former Labour transport secretary Lord Adonis said: "BAA has big explaining to do about its failure to open Heathrow properly for a third day running.
"It appears to have made totally inadequate preparations for winter snow and ice, and it is making Britain look like a third world country in terms of its transport systems."
Later, BAA said it had agreed a reduced flying schedule until 6am on Wednesday and that a "maximum one third of flights are likely to operate during this period".
BAA went on: "Passengers should anticipate further delays and cancellations in the following days and potentially beyond Christmas Day.
"Passengers should not travel to Heathrow without a confirmed booking. If they can travel at another time then they should do so."
BAA said it aimed "to provide passengers with better and more timely information". The company went on: "Airlines are currently updating which flights have been cancelled and http://www.heathrowairport.com is showing the latest available flight information.
"If the website shows 'contact airline' then the flight will not be operating today. Passengers must not travel to the airport unless they have confirmed that their flight is leaving today. Airline contact numbers are available on http://www.heathrowairport.com."
BAA said its northern runway at Heathrow was open and handling a limited number of arrivals and departures.
The company added: "Earlier in the day, we prioritised aircraft that had previously been diverted, in order to bring passengers to Heathrow and get airline crews and aircraft in the right place."
BAA said "several hundred" additional contractors had been brought in to clear the situation and the company had "deployed additional staff to provide comfort to passengers in its terminals".