Much of the UK was blanketed in heavy snow this morning as the extreme weather headed south and forecasters warned that the country was on course for its coldest winter in 30 years.

The Met Office issued an alert warning that nearly half a metre of snow was due to fall in some areas, while freezing conditions spread after having brought chaos to the north of England and Scotland today.

Tony Waters, the Met Office chief forecaster, said: "This is expected to cause disruption to transport networks and could lead to problems with power supplies."

A spokesman for the prime minister said the government was doing all it could.

"The weather is taking a turn for the worse," he said. "The Highways Agency has kept the vast majority of major road networks running. We are in close contact with local authorities and it is a situation we will keep a very close eye on."

Forecasters say that the cold snap, which began in mid-December, is the longest since 1981. And they warned there was no end in sight.

"I would normally be loth to look beyond five to seven days, but the way the conditions are set at the moment I think the cold weather is not going to change for some time," said Stephen Davenport, senior meteorologist at MeteoGroup. "I will stick my neck out and say it will be here for a couple of weeks and possibly longer."

If the freezing conditions continued for the rest of the month, he said, the UK would be on course for its coldest winter since 1979. The snow was expected to hit southern counties of England, with almost 40cm predicted to fall on Salisbury Plain overnight. Heavy snow was also expected across London and parts of Wales.

Waters said yesterday: "The heaviest snowfall this evening and tonight is expected across parts of Dorset, Wiltshire, Hampshire, Berkshire and Oxfordshire – where fresh snowfall of 15 to 20cm is expected widely, and locally in excess of 30cm. This is expected to cause disruption to transport networks and could lead to problems with power supplies."

The National Grid issued a rare warning to power suppliers to use less gas yesterday after a 30% rise on normal seasonal demand. It has urged power firms to switch to coal, and order more gas supplies from Belgium and Norway.

The business secretary, Lord Mandelson, told the BBC it was "a time of special pressure on the grid" but did not address claims from the Conservatives that Britain has only eight days' worth of gas storage left.

Meanwhile stockpiles of gritting salt held by councils for roads are running thin. Emergency deliveries were made to Fife council in Scotland, and in Wales Pembrokeshire council warned that gritting lorries were struggling to cope with the "extraordinary" conditions.

Derek Turner, network operations director at the Highways Agency, said: "We are working flat out to keep our roads safe and serviceable for use. However, it is very important to drive appropriately for the conditions; even when roads are treated and appear ice- and snow-free, they should still be negotiated with care."

Workers at Winsford rock salt mine in Cheshire said they were unable to meet the unprecedented demand despite pre-winter deliveries being completed.

"The worst continuous spell of severe weather for 20 years has led to massive additional demand. We would obviously like to be able to fulfil every authority's needs in full, but the reality of the situation at present is that that is simply not possible," the Winsford Salt unions said.

The mine is capable of extracting 30,000 tonnes of rock salt per week, but local councils in England are spreading that amount on the roads every day in an attempt to keep traffic flowing.

A Local Government Association spokesman insisted that councils, which are responsible for gritting many A roads as well as minor and town roads, were prepared: "As far as we are aware there are sufficient supplies of salt in the country to deal with the current cold snap. How much grit each council holds depends on local circumstances and how much bad weather they tend to get."