Drivers have been warned that there will be extremely difficult driving conditions tomorrow with heavy snow forecast.

A Met Office yellow weather warning for snow and ice has already been issued across the region.

Now, National Highways has issued a severe weather alert for snow in the south of England for Wednesday and Thursday – asking people to reconsider journeys on parts of its road network.

The weather warning means driving conditions will be extremely difficult, especially for larger vehicles and those not equipped with four-wheel drive between midday Wednesday and 3am on Thursday.

National Highways is asking everyone travelling in these areas to consider if their journey is essential.

HGV drivers are asked to avoid parts of the road network with steeper than normal inclines. This could potentially lead to their vehicle becoming jackknifed or stranded in the wintry weather.

Where vehicles get stuck it can prevent gritting vehicles from getting through to treat the road with salt or to plough snow clear from the road.

National Highways executive director of operations, Duncan Smith, said: "We are well prepared and our gritters have been out in full force since Friday and will continue to treat the roads.

"We are asking drivers to plan ahead, check the forecast and allow more time for their journeys.”

Roads that are thought to be at risk of highest impact are: M4, A303, A417 near Gloucester, M5 J31-22, M25, M3, M20, M23, A3, M40 J1-9, A38 Haldon Hill and A30 Okehampton.

Snow is expected in the morning, followed by a more significant second wave of snow in the afternoon and evening.

It could be prolonged and occasionally heavy, particularly during the evening rush and to the east of the M5, leading to potential slow traffic and loss of traction on steep inclines. The current estimate is for two to five centimetres of snow, and up to 10cm on higher ground and hills.

There could then be icy stretches during the morning travel period on Thursday, presenting another hazard to motorists.

National Highways, in conjunction with the Met Office, are strongly advising that travel in these areas is likely to be extended or even disrupted.

Road users should heed messages on the overhead signs and listen for radio updates.

Further information can be found by visiting http://www.trafficengland.com/ or calling the National Highways Customer Contact Centre on 0300 123 5000.