A health warning has been issued for the elderly and unwell as temperatures plummet and thick fog causes travel disruptions across the UK.

Public Heath England has issued a “level three” cold weather alert for the first time since January, and issued a stark reminder to the public that “cold does kill”.

The Met Office has meanwhile issued yellow “be aware” fog warnings across most of England and parts of Wales.

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.”

Mr Waite encouraged people to think about how to help vulnerable friends and family during the severe weather, warning that cold weather can be fatal.

“This is also the time to think about how the cold temperatures may affect your friends and family, particularly if they are older or very young or have long term health conditions as they are particularly at risk from the ill-effects of cold. Think now what you could do to help them,” he added.

In pictures: Winter weather hits the UK Show all 17 1 /17 In pictures: Winter weather hits the UK In pictures: Winter weather hits the UK A man works in the snow in the Yorkshire Dales near Hawes In pictures: Winter weather hits the UK A man jogs past the partially frozen Sefton Park Lake in Liverpool PA In pictures: Winter weather hits the UK A jogger runs through the snow under the Angel of the North in Gateshead, Tyne and Wear PA In pictures: Winter weather hits the UK A man walks through the snow close to the Angel of the North in Gateshead, Tyne and Wear PA In pictures: Winter weather hits the UK Temperatures drop as night time falls and snow blankets moors in the Peak District near Buxton Getty Images In pictures: Winter weather hits the UK Temperatures drop, as night time falls, and snow enhances the complex pattern of fields on the moors in the Peak District near Buxton Getty Images In pictures: Winter weather hits the UK A van waits to be recovered after sliding into a ditch on an icy road in the Peak District near Buxton Getty Images In pictures: Winter weather hits the UK Snow blankets the countryside in the Peak District near Buxton Getty Images In pictures: Winter weather hits the UK Farmers on the Richmondshire and Cumbria border take feed for the sheep on the Pennine tops with the A66 trans Pennine route in the background as snow falls across many parts of the UK PA In pictures: Winter weather hits the UK A tractor with a snow plough parked outside the Tan Hill Inn in Swaledale, North Yorkshire as snow falls in the Pennines PA In pictures: Winter weather hits the UK Cars make their way through the snow on the A1 northbound in Gateshead, Tyne and Wear, as the UK braced for a new wave of bad weather after forecasters issued warnings of heavy snow in parts of England and Scotland PA In pictures: Winter weather hits the UK Snowfalls over high ground of the Pennines at Tan Hill PA In pictures: Winter weather hits the UK Sue and Chris Betts take their dog for a walk in the snow near Beamish, Tyne and Wear after heavy snow in the area PA In pictures: Winter weather hits the UK Clyde Wind Farm near Abington in Scotland PA In pictures: Winter weather hits the UK Sheep near Abington in Scotland PA In pictures: Winter weather hits the UK Snow in Abington in Scotland as the cold weather hits parts of the UK PA In pictures: Winter weather hits the UK Snow blankets the countryside in the Peak District near Buxton Getty Images

“Cold does kill, even in places where the temperatures aren’t at their lowest. Most of the practical advice on keeping warm in cold weather may seem like common sense, but it’s important that we all help each other to stay well this winter.”

The Met Office stated on Thursday that the mist and fog would likely be “more widespread” into Thursday night, but that “increasing cloud” would help fog clear by Friday morning, with minimum temperatures of -2c.

It comes a day after a motorist was killed in a major crash in foggy conditions on the A40.

RAC spokesman Rod Dennis warned on Wednesday that motorists must take extra care when faced with “colder temperatures, ice and fog” on the roads.

Mr 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 four 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.”