For years, they have boasted of the health benefits of their leafy diets, but now vegetarians have the proof that has so far eluded them: when it comes to cancer risks, they have the edge on carnivores.

Fresh evidence from the largest study to date to investigate dietary habits and cancer has concluded that vegetarians are less likely to develop cancer of the blood than meat eaters and are less likely to develop cancer overall.

Scientists said that while links between stomach cancer and eating meat had already been reported, they had uncovered a "striking difference" in the risk of blood cancers including leukaemia, multiple myeloma and non-Hodgkin lymphoma between the groups. The study looked at vegetarians, fish eaters and people who ate meat.

Co-author Naomi Allen, from the Cancer Research UK epidemiology unit at Oxford University, said: "Previous research has found that processed meat may increase the risk of stomach cancer, so our findings that vegetarians and fish eaters are at lower risk is plausible. But we do not know why cancer of the blood is lower in vegetarians."

She said the differences in cancer risks were independent of other lifestyle factors including smoking, alcohol intake and obesity.

However, Allen urged caution over the interpretation of the findings. "It is a significant difference, but we should be a bit cautious since it is the first study showing that the risk of cancer of the blood is lower in vegetarians. We need to know what aspect of a fish and vegetarian diet is protecting against cancer. Is it the higher fibre intake, higher intake of fruit and vegetables, is it just meat per se?"

The study also reported that the total cancer incidence was significantly lower among both the fish eaters and the vegetarians compared with meat eaters.

The study, published in the British Journal of Cancer, is part of a long-term international study, the European prospective investigation into cancer and nutrition (Epic).

Today's findings were based on a study of 61,566 people who scientists followed over 12 years. During this time, it was found that 6.8% of meat eaters (2,204 of 32,403), 4.0% of vegetarians (829 of 20,601) and 3.7% of people who ate fish but no meat (317 of 8,562) were diagnosed with cancer.

They found that 180 meat eaters developed blood cancers, while 49 vegetarians developed the diseases and 28 fish eaters. They found the risk of being diagnosed with cancers of the stomach, bladder and blood was significantly lower in vegetarians than in meat eaters but, in contrast to earlier work, they found the rate of bowel cancer was slightly higher among vegetarians than meat eaters.

A spokesman for BPEX, the British pig executive, questioned the methodology of the study: "We are unable to take a view on this because there is mixed evidence based on the compounding factors to do with lifestyle that come into it."

Richard Lowe, the chief executive of Eblex, the English beef and lamb executive, said: "We think that the link between diet and cancer is complex and as scientists themselves say, more research is needed to see how big a part diet plays."

The Oxford research is the latest in a series of reports to discourage too much meat in the diet. Last year, Dr Rajendra Pachauri, chair of the UN Intergovernmental Panel on Climate Change – which last year earned a share of the Nobel peace prize – urged giving up meat at least once a week as a way of combating global warming. The UN's Food and Agriculture Organisation has estimated that meat production accounts for nearly a fifth of global greenhouse gas emissions.

Two years ago, the World Cancer Research Fund found a link between red and processed meat and bowel cancer and recommended that the average amount of meat eaten should be no more than 300g a week. In Britain, the current meat intake is about 970g a week for men and about 550g a week for women.

In 2005, the Epic study, funded by the Medical Research Council, Cancer Research UK and the International Agency for Research on Cancer, concluded that eating just two portions of red meat a day – the equivalent of a bacon sandwich and a fillet steak – increased the risk of bowel cancer by 35%. It found that eating fibre, in the form of vegetables, fruit and wholegrain cereals, lessened the risk of cancer and that fish, eaten at least every other day, was also protective.

Annette Pinner, chief executive of the Vegetarian Society, said: "It is widely recognised that a third of cancers are directly related to diet and what's interesting in this study is the findings on blood cancers. We wouldn't claim vegetarianism is a panacea for cancer but it is a step in the right direction."

• This article was amended on 12 October 2011 to correct figures from the study. The original said the study concluded "that vegetarians are 45% less likely to develop cancer of the blood than meat eaters and are 12% less likely to develop cancer overall". The article also originally said: "Today's findings were based on a study of 61,000 people who scientists followed over 12 years. During this time, 3,350 participants were diagnosed with cancer. Of those, 68% (2,204) were meat eaters, 24% (800) were vegetarians and 9.5% (300) ate fish but no meat." This has been corrected.