Scientists have bred cows that produce skimmed milk and hope to establish herds of the cattle to meet the demands of health-conscious consumers.

The milk is also high in omega3 oils, claimed to improve brain power, and contains polyunsaturated fat. The saturated fats found in normal milk are linked to increased risk of heart disease. The cows, which have a particular genetic mutation, were bred from a single female discovered by researchers when they screened milk from millions of cattle in New Zealand.

Butter from these cows has the extra advantage of being spreadable straight from the fridge, like margarine.

Scientists at ViaLactia, the biotech firm behind the £55m research, have named the cow Marge. Russell Snell, ViaLactia's chief scientist, said: "Marge looks like an ordinary Friesian cow but has three key differences. She produces a normal level of protein in her milk but substantially less fat, and the fat she does produce has much more unsaturated fat. She also produces milk with very high levels of omega3 oils."

Marge was discovered in 2001 when ViaLactia's researchers bought her from her owner for £120 and moved her to a secret location.

The key issue was whether her calves would inherit her traits. "You have to generate daughters and then they have to carry a calf and deliver milk," said Snell. "The eureka moment was when we found her daughters produced milk like their mother."

The Auckland-based company says the first commercial herds for spreadable butter could be expected by 2011.

A brief description of ViaLactia's research is due to be published this week in Chemistry & Industry, a journal of the Society of Chemical Industry. A formal research paper for a peer-reviewed journal will follow.

Britain produces 24.6 billion pints of milk a year of which 7.7 billion is for drinking. Growing health concerns mean that full-fat milk accounts for only a quarter of sales. The rest is semi-skimmed or skimmed, according to Dairy UK, the industry association.

ViaLactia hopes Marge's male offspring carry the same genes as her daughters. "To have a bull from Marge's offspring who passes on her traits would be the holy grail. It would allow us to reproduce hundreds of thousands of cows like Marge," said Snell.

The scientists are still trying to identify the genes behind Marge's unique traits. Klaus Lehnert, 43, Snell's deputy, said: "We do expect to find them. We are good at finding genes. Then we can use DNA tests to find if an animal has the trait, rather than rely on data from experiments."

Milk was once universally regarded as a health drink, thanks to heavy pro-motion by the government. Generations of children grew up with slogans such as "Drinka pinta milka day". Free supplies were given to schoolchil-dren and pregnant women. When questions began to be raised about the fat content of milk, the Milk Marketing Board switched to trying to sell milk as sexy, targeting housewives with slogans such as: "Is your man getting enough?"

Government health campaigns now push low-fat diets and sales of whole milk, which contains 3.5% butter fat, account for just 25% of milk sales.

By contrast, sales of semi-skimmed milk, which contains 1.7% fat, and skimmed milk, which has 0.1% fat, account for 75% of sales. The New Zealand animals are understood to have less than 1% fat in their milk.

Ed Komorowski, technical director at Dairy UK, said a proper evaluation depended on ViaLactia's research being published in a scientific journal.

"The New Zealand approach is exciting because people tend to avoid full-cream milk and go for semi-skim and skim. If whole milk can be made to contain unsaturated fats, which are good for you, then people may change back to whole milk," he said.

Dr Susan Jebb, head of the Medical Research Council's human nutrition unit, said such a milk could contribute to the nation's health. "Dairy products make a significant contribution to our saturated fat intake, which is already 30% higher than recommended. A milk that has less fat, and fats of a better type, would be a lot healthier."

Tom Brooksbank, of Norton& Brooksbank, one of Britain's leading livestock auctioneers and valuers, said animals able to produce such milk could command a premium. "A cow that produced milk low in saturated fats and high in healthy ones would be a big hit, especially if it was natural," he added.

Helen Wallace, of GeneWatch, the genetics watchdog, said it was important to ensure the mutations that produced the milk were not harmful in any way. "If so then using such animals would be far better than creating genetically modified cows."