Sponsors of the legislation that could block Wal-Mart and some other big-box stores from opening in the city said they may have to make amendments to the law even after the City Council voted yesterday to override the mayor’s veto and even before the law goes on the books.

An official at a trade group that represents local grocery chains including C-Town and Met Foods, the National Supermarket Association, told The New York Sun that its member businesses, many of which have retail spaces between 15,000 and 20,000 square feet, would be devastated by the law.

As passed, the law would require grocery stores that are at least 10,000 square feet or have 35 or more employees to contribute a “prevailing health care expenditure rate.”

“The amount of money that this will cost will put 40% to 50% of our people out of business,” the executive director of the trade group, Luis Salcedo, said during a telephone interview.

These markets appear to be unintentional victims of the law.

An amended version, likely to be introduced later this month, could increase the square footage and employee count thresholds and allow smaller supermarkets to continue doing business as usual.

The last-minute opposition to the measure seems to center on a communication breakdown over whether the NSA had a representative speaking on its behalf as the council was crafting the legislation. Mr. Salcedo said it did not, while the lead sponsor of the legislation, Council Member Christine Quinn, said her staff had been speaking with the group’s representative for the last year and a half and that no concerns were expressed. Ms. Quinn declined to name the representative, saying only that it was a government affairs professional.

“We think we did appropriate due diligence and outreach that yielded no input,” Ms. Quinn said. “But they have raised concerns, which we are going to give serious consideration.”

A spokesman for Mayor Bloomberg, Robert Lawson, said that with or without changes, the so-called Health Care Security Act violates federal law. The mayor has said that while he wants to get more people health insurance coverage, the council does not have the authority to regulate the plans that private businesses offer.

Other opponents have said that the bill was clearly aimed at keeping big box stores like Wal-Mart out of the city and that it was pushed by city grocers that already offer benefits and want their competitors to incur the same expenses.

Council officials estimate that grocers will have to pay between $2.50 and $3 an hour for every employee, but the exact figure has not yet been calculated. They say it will help thousands of uninsured workers and protect businesses that now provide health benefits.

Forty of the 51 council members voted in favor of the override. Council members Simcha Felder and Andrew Lanza voted against it, Charles Barron and Peter Vallone Jr. abstained, and the remaining members were absent for yesterday’s vote.

Mr. Lanza, one of three Republicans in the body, said the planned amendments further prove that the bill targets Wal-Mart.

“It seems to me that this bill provides health care benefits for employees who work at a store that doesn’t even do business in this city,” Mr. Lanza, said. “They seem to be waiting for the final measurements of Wal-Mart, maybe the door widths, the heights of their buildings, and the exact square footage, so that they can tailor this bill perfectly.”

Mr. Felder said he supports the concept of the legislation, but that as written, the law would hurt four major grocers in his Brooklyn district.

“The threshold is much too low, and it would hurt small businesses,” he said. “If they changed the thresholds, I would vote for it.”

Several members voted for the measure despite their concerns about how smaller supermarkets would be affected. Council Member Margarita Lopez, for example, said she was voting for it, but only because she had received a commitment from Ms. Quinn that it would be amended.

Council Member Hiram Monserrate, who voted for the bill, said he too had concerns because some of the smaller grocers, many of which are minority owned, have “a very small margin of profit.”

“Unfortunately, some of these concerns were raised very late in the process,” he said. Pat Purcell, a representative of the United Food and Commercial Workers, a union that backed the legislation, said the National Supermarket Association had its “head in the sand” and should have come forward with its concerns earlier.