Commuters have faced travel chaos on Monday morning as large parts of the UK were hit by ice, fog and snow.
The Met Office issued yellow severe weather alerts due to the wintry conditions.
It was the coldest night of the year so far with temperatures in northern Scotland dipping below minus 15 Celsius.
Among the locations with fresh snow on Sunday included Andrewsfield, Essex (9cm), Charlwood, Surrey (5cm) and Herstmonceux, East Sussex (4cm).
Roads in eastern and south-east England are among the worst affected.
Motorways are experiencing heavy delays. Daniel Duffield
Drivers on the M25 were stranded for several hours due to the UK’s busiest motorway being closed in both directions between Junction 23 for South Mimms and Junction 25 for Waltham Cross, both in Hertfordshire.
National Highways said the carriageway will be “cleared and treated as soon as possible”.
There are delays of up to 90 minutes on neighbouring stretches of the motorway.
Other roads in the area with long delays include the M11, M2, A21, A27 and A249, according to National Highways.
Sussex Police issued a warning for “treacherous” roads.
Daniel Duffield, a 22-year-old off-duty paramedic, was stuck on the M25 for more than two-and-a-half hours on Monday morning while attempting to travel home to Birmingham from London.
He told the PA news agency that vehicles have been “stationary with very small movement”.
Delays are occurring on the rail lines. James Manning
He went on: “I expected some traffic due to weather but nothing this bad.
“Traffic appeared to be at a standstill for miles and miles.
“Other drivers were turning their engines off presumably to save fuel due to the uncertainty.
“A few people were stretching their legs due to delays too.”
He added that the road “doesn’t appear to have been gritted”.
AA president Edmund King said the number of breakdown callouts is around 25% higher than normal.
He went on: “Many drivers were stranded or severely delayed on the northern section of the M25 last night between London Colney and the M11.
“Traffic is still severely delayed there this morning due to the continuous snow fall.
“Roads in Kent were also severely affected including the M2, and M20.
“The conditions show that it is essential to be prepared if you are driving.”
He urged drivers to check fuel levels and take warm clothing, a charged mobile phone, food and drink.
“The best advice if you must drive is to take it easy and leave a much longer distance from the vehicle in front,” he added.
Several rail lines are closed, with Southeastern issuing a “do not travel” alert to passengers.
Gatwick and Stansted airports closed their runways on Sunday to clear snow. Scott D'arcy
There was also major disruption to services run by Greater Anglia, South Western Railway and Southern.
Four children were taken to hospital in a critical condition after being rescued from a lake in Solihull in the West Midlands on Sunday afternoon after falling through ice.
Richard Stanton, area commander for West Midlands Fire and Rescue Service, said: “We know that the weather forecast for the next few days is expected to be bitterly cold, please, adults and children alike, stay away from open water, under no circumstances venture on to ice regardless how thick or safe you think this ice may be.”
Gatwick and Stansted airports closed their runways on Sunday to clear snow.
Around 90 departures were cancelled from Gatwick between midday and midnight on Sunday, with at least a further 37 axed on Monday.
EasyJet was the worst affected.
Both Gatwick and Stansted were open on Monday morning but passengers were warned about the potential for further delays.
London City Airport said it is “experiencing some disruption this morning” due to aircraft being out of position after the “significant amount of cancellations” on Sunday night.
The Met Office has also issued a weather warning for ice in eastern and south-east England until 11am on Tuesday.
There is also a warning for wintry showers in many areas of northern Scotland and parts of north-east England for 48 hours from midday on Tuesday.