Freezing fog will create dangerous driving conditions in England on Sunday, the Met Office has said, as the London Mayor warned the weather would exacerbate air pollution in the capital.
The Met Office has issued a yellow weather warning for fog covering much of England, including Yorkshire, the South East and the Midlands.
Travellers should plan for slower journey times by car, possible disruption to bus and train services and a chance of delayed or cancelled flights, forecasters said.
London Mayor Sadiq Khan warned pollution would intensify on Sunday as cold, foggy conditions trap in emissions from cars and vans that burn fossil fuels.
"On Sunday, alongside the extreme cold temperatures we've been experiencing, we are also expecting moderate to high levels of air pollution," he said.
The mayor has issued a "moderate" air pollution alert.
Air pollution increases the risk of respiratory infections, heart disease and lung cancer and tends to hit those who are already ill, children the elderly and poor people, according to the World Health Organisation.
The mayor urged Londoners to "look after each other by choosing to walk, cycle or take public transport, avoiding unnecessary car journeys, stopping engine idling and not burning wood or garden waste, all of which contributes to high levels of pollution".
"This is particularly important in order to protect those who are more vulnerable," he added.
The warning tops off an extremely cold week, when temperatures plunged below -10C in parts of the UK amid snow and icy conditions.
But Met Office meteorologist Craig Snell said the worst of the cold snap is over, with warmer temperatures on the way next week across the whole of the UK.
Mr Snell said: "Next week, looking at the severe front, it's looking pretty benign.
"We're starting to lose the risk of fog and temperatures are generally around where they should be.
"We'll probably lose the really hard frosts. In terms of ice and snow, it certainly looks like we're over the worst.
"We've got to keep an eye on risk of fog generally this cold spell, although the main hazards from it look like they are beginning to diminish."
The change will come as the Arctic blast that swept across the UK in the past five days gives way to warmer air from the Atlantic, but will take longer to reach southern areas.