Freezing fog played havoc with travel today, with flights cancelled, roads jammed and delays on main line rail and Tube services.
And forecasters warned that although warmer conditions were on the way, they would be accompanied by heavy rain.
The worst of the fog was lifting during the morning, with Heathrow having earlier cancelled around 60-70 flights.
London City Airport also had to axe six early-morning flights, as UK temperatures dipped as low as minus 10C (13F).
“We expect the fog to lift between 9am and 10am. Visibility is already improving,” said a Heathrow spokeswoman.
She went on: “There have been some delays as well as cancellations. There is bound to be a bit of a knock-on effect during the day. Also, some European airports are also affected by the cold weather.”
Cancelled flights at Heathrow included scheduled trips to Warsaw, Nice, Stockholm, Aberdeen and several German cities.
There was congestion on a number of major road routes, while a section of the A1(M) between junctions 39 and 40 in Yorkshire was closed.
On the railways, overhead wire problems caused 50-minute delays in the rush-hour between Bedford and Luton, while a broken-down train at Watford Junction in Hertfordshire also caused hold-ups.
An electrical supply problem led to delays to train services between Seaford and Lewes in East Sussex, while a signalling problem caused hold-ups to services in and out of another Sussex coastal station – Brighton.
In Scotland, a broken-down train at Bathgate led to delays between Edinburgh and Airdrie.
On the Tube, there were severe delays on the Metropolitan line between Rickmansworth and Amersham and minor delays on the rest of the line due to a lack of trains.
There were also severe delays between High Barnet and Finchley Central on the Northern line due to a signal failure at Woodside Park.
Another signal failure – at Arnos Grove in north London – led to delays on the Tube’s Piccadilly line between Arnos Grove and Cockfosters.
The coldest spot in the UK last night was Tulloch Bridge in the Highlands, where the thermometer could only creep up to minus 10.2C (13F).
Other overnight cold spots included Chesham in Buckinghamshire at minus 9C (16F) and Little Rissington in Gloucestershire where it was minus 8C (18F).
“Some places where the fog lingers will struggle to get above freezing today,” said Matt Dobson, a senior forecaster with the Press Association’s weather division, MeteoGroup.
He went on: “London and the South East, where the sun will eventually come through, may reach around 3-4C (37-39F).
“But in places such as the Vale of York and in the Scottish Glens it may stay as cold as minus 3C (27F) in daylight hours.”
Mr Dobson continued: “This cold spell won’t last. The fog will lift. But although the big change on Thursday night, with an Atlantic front moving in, will mean higher temperatures, there is likely to be a lot of wet weather.
“The south of England could be very wet on Friday and we are likely to see wet conditions all the way up to Christmas.
“The exception could be a short, cold snap in the weekend before Christmas.”