Freezing fog has played havoc with travel, with flights cancelled, roads jammed and delays on main line rail and Tube services.
And forecasters have warned that although warmer conditions are on the way, they will be accompanied by heavy rain.
The worst of the fog lifted during the morning, with Heathrow having earlier cancelled 60 to 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, adding: "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."
The coldest spot in the UK on Tuesday 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," said Matt Dobson, a senior forecaster with MeteoGroup. "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."
Meanwhile, 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.