Snow could be on its way for people living in Wales and the far West of England.
Over night, wintry conditions are expected to affect higher altitudes in the west of the country, with some snow showers also hitting parts of The Pennines and the North York Moors.
But forecasters expect snow to spead to lower areas of Wales and the South-West peninsula by tomorrow morning.
Rachel Vince, forecaster with Meteogroup, the Press Association's weather division, said: "It should affect lower areas by tomorrow, especially in Wales, including some of the bigger population centres.
"The question is how far east this will go.
"Many people starting the working week probably will have some snow."
But she said it was unlikely any snowfall at lower levels would be substantial.
The likely limit of the snow showers is a line between Liverpool and the Isle of Wight but forecasters say the exact extent is difficult to predict.
The Met Office issued an amber warning of snow for parts of Wales, saying the hilly parts of the south and south east of the country could get as much as 10cm.
It said higher areas of south-west England, especially Exmoor, could see 5cm of snow but lower areas are more likely to see a 1-2 cm "slushy covering".
The Met Office today also issued a warning of severe cold weather across much of England until at least Thursday.
The next few days look like being the longest spell of cold weather so far this winter.
Forecasters say temperatures will drop as the week progresses with widespread frosts at night and low temperatures during the day but only the occasional wintry shower.
Police in Devon and Cornwall are warning motorists in some parts of the region not to travel unless it is essential.
There is an amber warning for snow that may make driving difficult on the higher parts of Exmoor and Dartmoor, a police spokesman said.
He added at this time they are having reports of snow causing some difficulty in Princetown on Dartmoor and the Lynton area of Exmoor.
A small amount of snow may occur in other areas of Devon, mostly on higher ground, but forecasts suggest that this will not cause significant disruption, the spokesman said.
Inspector Andy Oliver, from the police control room at Exeter, added: "Previously we have had problems on the A38 and A380 around Haldon Hill near Exeter.
"The Met Office advise us that although this area may experience a little sleet and/or snow it is unlikely to be adversely affected.
"Drivers should take care, slow down, listen to updates on local radio stations and give themselves extra time to get to their destination."
Gloucestershire County Council said that with bigger salt supplies than ever at this time of year it is ready for the forecast cold snap.
The county's gritters will be out from 9pm salting the key routes.
They will then salt the secondary route network from midnight and carry out an inspection at 3am to determine if any further action is required, the council said.
Councillor Will Windsor-Clive, cabinet member responsible for roads, said:"Even though it has been a mild winter so far we are always ready and prepared for snow and ice with a team of gritters on standby and back up teams and supervisors in each of the depots.
"The council has increased its salt stocks once again this year by opening two new salt domes - one in Cannop and one in Stroudwater depots which are operational now."
Drivers in the county can receive updates online about which roads have been gritted after the council set up a Gritter Twitter feed on the social networking website, Twitter, via @GlosHighways.
The service will be used to tweet regular updates on the weather, road conditions and gritting activity.
"When we have severe weather people want to be reassured that we are out treating the roads, where we have been and when," Mr Windsor-Clive said.
The council said parts of Gloucestershire could see accumulations of one to three centimetres of snow.
The two new domes have boosted salt levels from 9,600 tonnes last year to just over 11,000 tonnes this winter.
Nick Peters, Gloucestershire highways duty manager, said: "We are always prepared for bad weather.
"Soon after each winter finishes we start preparing for the next one.
"This includes reviewing the effectiveness of how we performed and where we can make changes to improve the way we deal with severe winters in the future."