Maine Gov. Paul LePage in June 2013.   (Robert F. Bukaty/AP)

Maine will become the second state to require labels on food that contains genetically modified ingredients under new legislation signed by Gov. Paul LePage (R) this week — but only after other states follow suit.

LePage signed the legislation, initially introduced by a Republican state representative, over the objections of agriculture giants who produce many of the raw ingredients that go into everyday foods.

The U.S. Department of Agriculture estimates that about 70 percent of the food products sold in supermarkets contain genetically modified ingredients, the Portland Press-Herald reported.

But shoppers in Maine won’t see those GMO labels slapped all over grocery stores any time soon. The legislation doesn’t go into effect until five nearby states, including New Hampshire, pass similar labeling laws. New Hampshire’s legislature will take up a similar measure during its legislative session this year.

That provision was necessary, the bill’s backers said, to build a broad base of support. It’s similar to a provision in a GMO labeling bill passed by Connecticut’s legislature, signed into law last month by Gov. Dannel Malloy (D), which won’t take effect until a combination of Northeastern states that add up to 20 million residents pass similar legislation.

Industry groups and mega-corporations such as Monsanto and General Mills are likely to challenge the new law in court. Monsanto has said it would file suit on the grounds that the law violates free speech and interstate commerce protections, the Press-Herald said.

Monsanto has threatened to sue other states over labeling legislation; in 2012, the threat of legal action effectively halted a GMO measure that had been advancing through the Vermont legislature. A ballot measure that would have required labels on genetically modified food in Washington state failed in 2013, after food producers spent more than $20 million on campaign advertising.

Though Maine and Connecticut are the only two legislatures to pass GMO labeling bills, similar legislation has been introduced in about 30 states, according to the National Conference of State Legislatures.