Britons have been warned to brace themselves for an increase in food prices as plunging temperatures leave farmers unable to harvest vegetables and hauliers struggle to distribute fresh produce.

Gordon Brown, who will chair a meeting of the Cobra emergency committee early this week to discuss the freeze, was today forced to reassure the country that it would not run out of gas or grit for its roads during the coldest weather in 30 years.

Police confirmed today that the weather-related death toll had risen to 26. A 90-year-old woman froze to death in her garden near Barnsley after falling in the snow. Widow Mary Priestland was discovered when her neighbour called round to make her tea. A 42-year-old Newcastle woman died after being found lying in the snow this morning. She had told her family she was going for a walk at 7pm on Friday.

Concerns have now switched to food supply. Sub-zero temperatures have made it impossible to extract some vegetables from the ground. Producers of brussels sprouts and cabbages are all reporting problems with harvesting. Cauliflowers are said to have turned to "mush" in the sustained frost, with the result that only imported ones are available – at more than £2 each.

"Food is selling fast and there is a problem with replenishing it," said Stephen Alambritis of the Federation of Small Businesses. "One business I spoke to said it was like Christmas Eve, with people rushing to buy up food. This will inevitably have an impact on food prices."

Food prices had already started to edge up after a sustained period of low inflation. Food inflation increased by 3.7% in December, up from 2.8% in November, said the British Retail Consortium.

In Ireland, 6,000 acres of potatoes remains unharvested and there are claims that up to three-quarters of the crop may be ruined. Potato growers in Northern Ireland say they are facing some of the biggest losses in recent history because of frost damage.

Meanwhile, greengrocers in some of the worst-hit areas are reporting shortages, with the price of carrots and parsnips reportedly rising by 30% in some small shops. A spokesman for the National Farmers' Union said: "There are isolated examples of farms struggling to get milk supplies out, but so far the majority of farmers, although finding it difficult, are getting on with the job." Milk suppliers in Somerset said they feared they may have to dump 100,000 litres of organic milk because tankers could not get through.

In a move that underscores the severity of the situation, on Monday the government will permit an emergency relaxation of European laws regulating the driving hours for hauliers involved in the distribution of animal feed. Under the temporary rules, the hauliers will be allowed to drive for 10 hours rather than the EU maximum of nine. There will also be a reduction in their mandatory daily rest requirements, from 11 to nine hours.

Today, the prime minister insisted gas supplies were not running out, despite record levels of demand. In a podcast from Downing Street, Brown said: "I can assure you: supplies are not running out. We've got plenty of gas in our own backyard – the North Sea – and we also have access to the large reserves in Norway and Netherlands."

Last week, nearly 100 large businesses were forced to stop using gas in an attempt to conserve supplies.

Brown also tried to allay concerns over salt stocks. "Working with the suppliers and the highway authorities, we are making sure stocks of salt to grit roads and pavements get to where they're most needed," he said.

On Friday, a government emergency planning committee met to discuss the UK's state of preparation if the cold weather continues. The committee heard the country has a stockpile of 320,000 tonnes of gritting salt, but transport sources suggest the UK is getting through 60,000 tonnes a day, more than four times the amount produced.

Further supplies are due to arrive from Spain and the US later this month. However, some have questioned how the UK will cope if these supplies prove insufficient and the cold snap returns. "The government has failed to build up a strategic Highways Agency reserve and ministers have sat on their hands," said the Conservatives' local government spokeswoman, Caroline Spelman.

Edmund King, president of the AA, said he had raised concerns about salt supplies before Christmas. "We were not assured that everyone was as prepared as they should have been… and that's why I wrote to the Local Government Association in November, prior to the cold snap," King told the BBC. "There is more we could have done."