The review by WHO's International Agency for Research on Cancer (IARC), released on Monday, said additionally that there was some link between the consumption of red meat and pancreatic cancer and prostate cancer.
IARC classified processed meat as "carcinogenic to humans" on its group one list along with tobacco and asbestos, for which there is "sufficient evidence" of cancer links.
Each 50-gram (1.8-ounce) portion of processed meat eaten daily increases the risk of colorectal cancer by 18 percent, the agency estimated.
A 50-gram portion would be the equivalent of eating one hot dog or two slices of bacon. Americans eat about 21.7 grams of processed pork per day, according to a 2011 National Health and Nutrition Examination Survey.
Red meat was classified as probably carcinogenic in IARC's group 2A list, joining glyphosate, the active ingredient in many weedkillers.
The IARC examined some 800 studies during a meeting of 22 health experts earlier this month.
"For an individual, the risk of developing colorectal (bowel) cancer because of their consumption of processed meat remains small, but this risk increases with the amount of meat consumed," Dr Kurt Straif of the IARC said in a statement.
The classification for red meat, defined as all types of mammalian meat including beef, lamb and pork, reflected "limited evidence" that it causes cancer. The IARC found links mainly with colorectal cancer—which is a cancer that starts either in the colon or rectum—but also observed associations with pancreatic and prostate cancer.
Inconclusive evidence of a link between processed meat and stomach cancer was also observed, it said.
The news prompted animal rights activists People for the Ethical Treatment of Animals to offer free vegan diet starter kits and outraged agriculture groups.
The Canadian Meat Council, which represents meat packers such as Maple Leaf Foods and the Candian-based units of Cargill Ltd. and JBS SA, rejected the findings as simplistic, while trade group North American Meat Institute said the IARC report "defies common sense."
Some scientists and researchers said the news may not add much to long-standing health recommendations to limit consumption of such meat.
The IARC does not compare the level of risk associated with different substances in a given category, so does not suggest eating meat is as dangerous as smoking.
"Nobody is telling people not to eat meat," said David Wallinga, senior health officer for health and environmental advocacy group Natural Resources Defense Council. "What they're saying is if you eat it, eat less of it and buy it from sources that have produced it better."
Meat-heavy diets
Meat suppliers and processors argue that meat provides essential protein, vitamins and minerals.
Shares of most meat companies were little changed on Monday. Tyson Foods Inc fell after a rating cut by JP Morgan to "neutral," which focused on the company's shrinking market share in packaged meat.
"I'm not expecting that report to be a big issue going forward," said Brian Weddington, a vice president of Moody's Investors Service's corporate finance group. "I think there's still going to be a lot of hot dogs sold tomorrow."
The medical community has long thought there could be a link between red or processed meat and colorectal cancer, said Tim Key, a professor at Oxford University.
"Eating a bacon bap every once in a while isn't going to do much harm; having a healthy diet is all about moderation," Key said in a statement from charity Cancer Research UK.
According to estimates cited by the IARC, 34,000 cancer deaths per year worldwide are attributable to diets high in processed meat. About 1 million cancer deaths per year are due to tobacco smoking, it said. —Reuters
