HEAVY fog which has caused travel chaos across the UK is expected to shroud parts of the country again tomorrow, forecasters said.

A major crash on the A40, in which one female driver died and around a a dozen were injured, happened after foggy conditions made it "really dangerous", one witness said.

It is thought there were at least four collisions with a number of people being seriously injured.

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A motorist on the A40 who witnessed the fatal pile-up said that the poor visibility meant it was difficult for cars to see each other before they were "on top of it".

A Met Office spokeswoman said the low visibility and frosty conditions were caused by clear skies and dropping temperatures.

She added: "As we go through the next couple of days we will see less fog and frost as the weather gets cloudier."

Dozens of flights were delayed or cancelled due to poor visibility at London City Airport on Wednesday morning, while a string of arrivals were diverted elsewhere.

Today the MetOffice issued a fog warning valid from 5pm today until noon tomorrow, saying: "Freezing fog patches will become more widespread and locally dense during Wednesday evening, overnight and through Thursday morning.

"Driving conditions will be difficult with journeys likely to take longer than usual whilst some delays to air travel are possible.

"Whilst not all areas will have fog, where it does form the visibility could be less than 100 metres. Combined with temperatures below freezing, there is also the risk of ice forming on some untreated surfaces particularly later in the night and early on Thursday morning.

"The fog patches are expected to slowly thin during Thursday, although some patches may persist throughout the day - most likely in a zone from the West Country to Eastern England."

The warning applies to the council areas of Wiltshire, Swindon, Gloucestershire, Bath and North East Somerset, Bristol, North Somerset, South Gloucestershire, Somerset, Devon and Dorset.

RAC spokesman Rod Dennis said: “Drivers are facing a mix of hazards – notably much colder temperatures, ice and fog that is slow to clear.

"Lingering fog calls for drivers to adjust their speed and maintain longer stopping distances. Use dipped headlights, but don’t be tempted to use full beam – thick fog simply reflects the light back making it even harder to see.

"Only use your car’s fog lights if visibility is badly reduced, as using them when it’s not means you risk dazzling other drivers.

"Motorists should remember that if their car thermometer records a temperature of 4 degrees or lower, there is a good chance the road temperature is near or even below freezing, meaning a very real risk of ice and slippery roads.

"Keep listening to traffic reports before you start your trip and, if conditions are bad, aim to stick to major routes which are much more likely to be gritted."

At London City Airport visibility on the airfield dipped as low as 50 metres today, a spokesman said.

He added: "We have had several delays and cancellations and some arrivals diverted as well.

"We were able to to operate seven departures successfully, but the airlines have had to cancel around 22 departures. On arrivals, we have had around 14 cancelled."

He added that they expected the fog to affect flights until around 2pm, with knock-on disruption continuing throughout the day.

Passengers were advised to check in advance with their airlines to see if they are likely to face delays, visit the airport's website or talk to staff on the ground.

Dr Thomas Waite, consultant in extreme events and health protection at Public Health England, said: "The effects of cold can be severe, in particular for those who are over 65, have a long-term illness, or are not mobile. Our advice to these groups is when indoors, have plenty of warm food and drinks and try to maintain indoor temperatures to at least 18C.

"If mobility isn't an issue, keep active as best you can. If you need to go out wear lots of thin layers and shoes with a good, slip-resistant grip to prevent any accidental falls.

"It is particularly cold at night this week, so drawing the curtains at dusk will keep the heat in."

He also stressed the importance of checking on potentially vulnerable family and friends during the cold period.