Freezing weather and thick snow continued to cause travel chaos, as police struggled to cope with a series of accidents in North Yorkshire as a result of the "treacherous" conditions.
No-one is thought to have been hurt but a 25-mile (40km) long section of the A1 northbound was shut this morning after a number of collisions between Allerton Park, near York, and the Catterick area.
A police spokesman said: "Traffic conditions on the A1 northbound from Dishforth and all the way to Leeming and beyond are treacherous.
"There have been a serious of accidents which have resulted in the A1 northbound being shut while emergency services clear the scene."
The spokesman said the incidents involved jack-knifed lorries and other vehicles which had taken evasive action. He said it was not known how long the clear-up operation would take.
North Yorkshire Fire Service said the multiple incidents started at about 2.15am and advised motorists not to travel on the A1 until further notice from the police.
The Highways Agency said the A1 at Catterick was closed southbound between the A6136 and A684, while the A1M in North Yorkshire was closed northbound between junctions J47 and the A6136.
Nine severe weather alerts were in place today as the Met Office warned that ice could be a hazard on roads and pavements across much of England and south-east Wales.
Issuing yellow alerts, which urge people to "be aware", it warned of "risk of disruption to travel" today.
Some rail services continued to suffer following the weekend's big freeze.
Southeastern hoped to operate a normal timetable, but said icy conditions and frozen snow cancellations and delays today were "very likely".
In a statement, the operator said: "These low temperatures are set to continue for several days. Please take care when travelling and allow additional time for your journeys."
Sub-zero temperatures caused havoc over the weekend.
Heathrow Airport axed half of all flights last night but the airport, which had 2.4in (6cm) of snow, hoped to resume normal service today.
A spokeswoman said: "We are expecting everything to operate as our normal schedule. There may be some cancellations and reshuffling as a result of yesterday's disruption."
On Saturday night up to 6.3in (16cm) of snow swathed much of the country, creating treacherous driving conditions and disrupting rail, road and air travel, while thousands of Britons ventured outside to enjoy the picturesque scenes.
But as the snow melted and the runways, taxiways and stands at Heathrow were cleared of snow, only 50% of the 1,300 scheduled flights were going ahead yesterday, leaving scenes of confusion and frustration for travellers who faced a night sleeping on terminal floors.
But Heathrow insisted its "snow plan" had worked "far better" than in previous years, saying the move was designed to minimise disruption as staff worked to clear a backlog of flights.
"We took the decision with airlines and air traffic control yesterday to reduce the flight schedule in advance," a spokesman said.
"By cancelling flights in advance, airlines have been able to rebook some people on to flights that are departing, and passengers have had better quality information about whether they can fly or not."
Transport Secretary Justine Greening defended the airport's cancellation strategy and said bosses had taken the "right approach".
The airport faced heavy criticism following severe weather in December 2010 when it almost ground to a halt.
At the height of the chaos on 19 December, it was able to handle only around 20 flights and thousands of passengers were forced to camp overnight in terminals.
A BAA-commissioned report later concluded that the operator's response was "initially ineffective" and that the potential impact of the weather had not been fully anticipated.
"Actually cancelling flights in advance so passengers don't get to the airport and then find their flight being cancelled was one of the main recommendations of the inquiry that Heathrow held into the debacle last year when we saw huge disruption," Ms Greening said.
Forecasters warned of icy conditions today, as temperatures hovered around freezing in most areas early today, although Church Fenton in Yorkshire recorded minus 8.6C (16.5F) with visibility less than 100 metres.
Billy Payne, forecaster with MeteoGroup, the weather division of the Press Association, said the worst of the weather was over, although ice had formed overnight and there was a risk of freezing fog.
"We had a partial thaw yesterday and cold overnight temperatures which meant we have had some ice form as a result.
"We had a bit of rain as well falling on some frozen surfaces which has exacerbated the ice risk a bit.
"But there's not been much in the way of snow."
Temperatures in East Anglia and south-east England are likely to hit 1C (33.8F) or 2C (35.6F) today but the temperatures will increase further west, with Cornwall, Wales and Northern Ireland set for up to 10C (50F).
The South of England will see cloudy skies today, with Scotland and northern England in line for sunshine and dry weather if the fog lifts, Mr Payne said.
However, he said there is "not much sign" of the weather turning warmer over the next 10 days.
A broken-down train and electrical supply problems in the Portsmouth area caused delays, disruption and cancellations to South West Trains services.
Passengers had to switch to buses to get to Portsmouth, while rail services on the busy routes to and from London Waterloo were hit all morning.
Giving more details about the problems on the A1 in North Yorkshire, police said they began at 2.15am near Bedale when an HGV left the road due to icy conditions.
A spokesman said this was very quickly followed by a number of other reports of incidents on the A1 throughout the whole North Yorkshire police area - between York and Darlington.
He said one HGV jack-knifed on the northbound carriageway at Hackforth and then two further lorries jack-knifed north of this along with a further one at Leeming which suffered a ruptured diesel tank as a result.
A number of cars were also affected by the weather conditions and police attended several reports of vehicles which had left the road at locations including Kirby Fleetham, Fencote and Leeming Bar.
Other incidents were also reported on the southbound carriageway.
The spokesman said: "Resources from the police, the fire service, ambulance and the Highways Agency all worked together to ensure that any occupants of the vehicles involved were safe and, despite the number of reports received, there were very few injuries reported, none of which were serious or life- threatening."
He said the Highways Agency had salted the roads in this area last night but unexpected rain showers washed it away.
The fresh rain froze due to low ground temperatures, leaving the road covered in sheet ice which "contributed to the collisions", he said.
Around 20 primary and secondary schools in Kent and Medway were closed or shut for part of the day.
East Sussex County Council said it had 40 primary and secondary schools closed or partially closed. In Brighton and Hove and West Sussex, all schools were open.
Although many parts of East Sussex, particularly on higher ground, were covered in snow and ice, there were no major accidents on the roads, according to the emergency services.
An East Sussex Fire and Rescue Service spokesman said: "Drivers appeared to heed the advice to stay off the roads over the weekend, with East Sussex Fire and Rescue Service reporting no major incidents in terms of road traffic collisions.
"Heavy snow and sub-zero temperatures meant that many stayed tucked up warm and safe inside."
Cross-channel ferry services from the Port of Dover in Kent were running normally, with sea conditions described as slight with good visibility.
Just four schools in the North East were closed due to bad weather.
Faults to the heating systems forced closures at Holywell Village First School, near Blyth, Northumberland, Hilton Primary School in Blakelaw, Newcastle, and St Joseph's RCVA Primary in Coundon and Bowes Hutchinson's CofE School, both in County Durham.
The weather affected no schools in Sunderland, North or South Tyneside, or Gateshead.
A Sunderland City Council spokesman said: "The weather's not too bad up here at the moment."
Email us your snow pictures, using the hashtag #snow to ukpicturedesk@teamaol.com