The debate over labeling genetically modified foods has shaped up to be one of the costliest initiative fights ever in Washington state, with most of the dollars coming from out of state.

Five corporations and a trade group representing food manufacturers have largely financed efforts to defeat Initiative 522, raising $17.2 million so far, according to the latest campaign finance reports. Supporters including Dr. Bronner's Magic Soaps and natural food companies have raised $4.7 million.

On Nov. 5, voters will decide whether to approve I-522, which requires genetically engineered foods offered for retail sale to be labeled as such. Products would have to carry a label on the front of the package disclosing that they contain GE ingredients.

Supporters say consumers have a right to know whether foods they buy contain such ingredients and a GE label is no different from other food labels. Opponents say it would cost farmers and food processors, and such a label implies the food is somehow less safe.

Genetically modified foods have been controversial for years, but the issue has gained renewed attention as states consider bills mandating labels for foods that contain genetically modified organisms, or GMOs.

"This year, we just saw an explosion with 95 bills in 28 states," said Doug Farquhar, program director for environmental health at the National Conference of State Legislatures. Connecticut passed a GMO-labeling law, but it's dependent on four other states passing similar legislation.

In California last year, voters narrowly rejected a GMO-labeling measure after opponents mounted a $46 million defense. Proponents raised $9.2 million, according to MapLight, a Berkeley-based organization that examines the influence of money in politics.

Many of the top donors in that California fight are now writing hefty checks to influence the Washington ballot measure.

The money raised so far by both sides, about $21.9 million, is the second highest amount for a state ballot measure, according to records kept by the Washington Public Disclosure Commission. It trails money raised for and against a 2011 measure to privatize liquor sales.

Nearly all of the opposition money against I-522 has come from six out-of-state contributors that also were among the top donors against California's measure. The Grocery Manufacturers Association has given $7.2 million, while the biotechnology company Monsanto Co. has given $4.8 million. The average contribution to No on 522 is $1.2 million.

About 72 percent of the money raised by supporters of I-522 has also come from out of state. Dr. Bronner's Magic Soaps has given the most: $1.7 million. The pro-labeling group has received lots of smaller donations from within the state. The average contribution to Yes on 522 is $874.

In television ads featuring Pike Place Market fishmongers and others, supporters say more information is needed so consumers can make informed decisions.

"We should be able to decide for ourselves what we want to buy and eat," said Trudy Bialic, public affairs director for PCC Natural Markets, a Seattle-based natural food retail cooperative that helped write the measure. "Without labeling, they're not providing full disclosure."

Other advocates worry about potential environmental harm from genetically modified crops, such as cross-contamination with non-GMO crops, while some object to increased corporate control of food.

Labeling foes say those who don't want GMO foods can buy certified organic products.

In TV ads featuring a farmer and a former state agriculture director, opponents say the measure creates misinformation by exempting many food products and poses unnecessary burdens for farmers and food manufacturers.

"To put a warning on it implies there's something wrong with it," said Dana Bieber, a spokeswoman for No on 522. "It's meant to alarm the customer."

An analysis prepared for the Washington Research Council and paid for by the opposition group estimates that mandatory labeling would cost an average family of four more than $450 a year after 2019. Northbridge Environmental Management Consultants prepared the report and a similar report in California; the figure assumes manufacturers will switch to organic or non-GMO ingredients because they would not want to use the GE label.

The state Office of Financial Management estimates it will cost $3.4 million over six years to implement the measure.

Genetically modified crops come from seeds that are engineered to resist insecticides and herbicides, add nutritional benefits or improve crop yields.

Most GMO crops such as field corn and soybean are used for animal feed or as ingredients in processed foods including breakfast cereal, potato chips, baked goods and sodas. Under I-522, seeds or foods containing GMO ingredients would require a label starting in 2015. Some foods are exempt, including restaurant food, alcohol, certified organic food and medicine.

The Yes on 522 campaign has steered cleared of raising safety issues, but the initiative text says mixing plant, animal and other genes "can lead to adverse health or environmental consequences."

The World Health Organization has said GMO foods currently available on the international market "are not likely to present risks for human health" and no ill health effects have resulted. The American Medical Association sees "no scientific justification for special labeling of bioengineered foods" but favors mandatory testing before they hit stores.

With the potential for genetically modified animals to be approved for eating, advocates say there's even more reason to label. The Food and Drug Administration is considering whether to approve the first genetically altered animal for human consumption, a farmed Atlantic salmon that grows twice as fast as normal.

"If they want to eat GM salmon, that's fine. It's their choice, but we want the information to be out there," said Pete Knutson, owner of Loki Fish Co., which markets its wild caught salmon at farmers markets in Seattle.

Critics say some labeling advocates, including natural or organic producers, have their own motives: to drive consumers to their own products.

Major agricultural groups including the Washington State Farm Bureau, Washington Association of Wheat Growers and Washington State Dairy Federation have lined up against the measure, while the United Farm Workers, the Washington State Nurses Association and Washington Conservation Voters are in support.