Brits are being urgently warned of blankets of fog covering the country which could lead to widespread travel disruption.
The Yellow Weather Warning issued by the UK's Met Office is the second this week after a two-day threat of thick fog causing travels chaos between Monday and Wednesday.
It runs from 5pm today to 11am Friday.
Forecasters now say that disruption is going to last longer - between today and Friday - with thick blanketing fog incoming as temperatures plunge to lows of -4C.
The weather agency warned Brits of the "extreme danger" travelling in fog presents as it stifles visibility.
People are being advised to "avoid travel" completely if possible. But if on the road, motorists should approach driving with major caution and use "dipped headlights". Full-beam lights should be avoided because they reflect off the fog - causing a "white wall" effect.
A map of the affected area shows the thick fog covering huge swathes of England, including the Midlands, Bath and even bursting through Reading and into parts of West London.
At its most northerly point it reaches towards Liverpool after passing Birmingham and Stoke-on-Trent, while at its most southern point it careens along the south coast, engulfing coastal areas like Portsmouth and Brighton.
Explaining the warning, the Met Office wrote: "Fog patches will form during the evening and then become more widespread overnight - perhaps leading to some travel disruption."
Forecasters said Brits should expect "slower journey times" with bus and train service delays "possible".
There is also the risk of flights being delayed or cancelled at airports lying in the fog's path.
Airports in the affected areas include London Stansted and Birmingham International.
The fog is paired with plunging ice-cold temperatures, with forecasters calling the combination "dense freezing fog".
Issuing further fog travel advice, the Met Office say it's important to watch your speed as the weather form can "give the illusion of moving in slow motion".
It's also important to remember to switch off fog lights after visibility improves, and to not rely on tail lights in front of your vehicle to judge your position, as they can give a "false sense of security" while in fog.
The national weather agency also warned of "freezing fog" made of water droplets that "freeze on contact with objects such as the pavement, road, car etc" because it can rapidly form layers of ice.
The current fog front is particularly risky due to predicted widespread frost and plunging temperatures, with the mercury expected to hit lows of -4C in some regions.
It comes as the UK faces a barrage of frosty weather with temperatures set to drop below zero and up to five centimetres of snow is expected to fall.
Maps from WXCharts show that the Atlantic pressure system that brought cold weather and snow from Greenland at the start of the month will continue to hit the UK, with temperatures dropping as low as -1C in parts at the start of this week.
Met Office forecaster Alex Deaking said earlier this week: " In Scotland and Northern Ireland there’s a hint of blue on the charts and so certainly in rural areas and hills there will be pockets of frost.
"Most towns and cities just about staying above but we are in the colder air here further south with the cloud and the rain we are a long way from freezing with towns in the south-west perhaps staying in double digits."
