For at least 20 years doctors have been urging their patients to eat more oily fish to benefit the heart. Adding two servings a week of mackerel, salmon and similar fish to the family shopping list was believed to help fend off cardiovascular disease.

Now a major new study suggests the advice was wrong. Scientists who reviewed 89 studies of Omega 3 fats found no clear evidence that they are of any use at all.

Researchers from the University of East Anglia and eight other institutions say that when the results were pooled they showed no strong evidence that Omega 3 fats had an effect on overall deaths, heart disease, stroke or cancer.

The finding, if confirmed, will place fish oils at the top of the list of medical shibboleths that turned out to be myths. Among them are claims that fibre can prevent bowel cancer, vitamin C can halt colds, spinal manipulation can cure back pain, tranquillisers can cure anxiety and removing tonsils can prevent throat infections.

All have held sway, in some cases for decades, leading patients to treat themselves or seek treatment that turned out to be worthless.

Sales of fish oil capsules have soared on the back of the advice for the millions who find oily fish unpalatable.

Eggs high in Omega 3 fats and margarine enriched with them have appeared on the market in recent years, in response to consumer demand.

The health value of these products is now in doubt. The British Heart Foundation responded to the unexpected result by calling for more research. The Health Supplements Information Service, representing manufacturers of fish oil capsules, suggested Omega 3 fats might affect different people differently. Only further studies could supply the answer, it said.

For their review of Omega 3 research, the scientists from the University of East Anglia selected studies that involved a treatment group and a control group and had investigated the effect of consuming extra Omega 3 fats on health for at least six months. Differences in the quality of the studies were taken into account to minimise bias. In 48 of the trials, the Omega 3 fats were taken in the form of dietary supplements such as capsules.

Until three years ago, cumulative evidence showed a benefit from Omega 3 fats, but a major study called the DART-2 trial published in 2003 changed the overall picture. It included more than 3000 men and showed a higher heart death rate in those taking fish oil capsules.

Many consumers take fish oil capsules, which are enriched with vitamins A, D and E, for their joints, skin and hair, but the study did not examine their effects on these.