Adverse weather conditions have caused problems at airports as several flights were grounded after ice and freezing fog descended on the UK.
Heathrow Airport said more than 20 flights due to take off on Wednesday had been cancelled. A spokesman said more cancellations and delays were expected as conditions worsened.
He added: "Some flights have been cancelled and some are being cancelled tomorrow morning because of fog. It's better to cancel flights when there's fog because in fog you can't land as many planes per hour because you have to leave more space between the planes."
The spokesman said cancelling in advance helped airlines accommodate passengers on other flights. Cancelled flights included scheduled trips to Warsaw, Nice, Stockholm, Aberdeen and several German cities.
There are also disruptions and cancellations at London City Airport. A message on the airport's Twitter page said: "Visibility has not improved, flight disruptions are present & expected to continue until further notice. Please call your airline for info."
Large swathes of the country are expected to experience adverse weather after one of the coldest nights of the year. The mercury plummeted to minus 10C (14F) in some parts of the country overnight on Monday, with Scotland experiencing the lowest temperatures.
Julian Mayes, forecaster for MeteoGroup, the weather division of the Press Association, said central England, Wales and East Anglia had been hit by fog, which is likely to spread through the night.
He said: "The fog is currently pretty widespread. It's currently located over central England as a large area of low cloud, which is descending down as fog. Visibility is poor and the areas affected include the Midlands, the eastern side of Wales, most of East Anglia, most of the area just to the north of London, Bristol and parts of central southern England.
"Through the night it's going to return to Somerset and become a bit more widespread in valleys across Wales. The far South East might miss it, although the London suburbs may become affected later on tonight."
Mr Mayes said in the worst affected areas there will be less than 100-metre visibility. The weather is expected to get worse, with rain and snow predicted on higher ground. He added: "When the fog lifts the weather is going to become a little more unsettled for the second half of this week with rain. Probably no significant snow but it will end up wet and windy by Friday, so a big change is coming."