Under pressure from states saddled with rising Medicaid costs and from labor unions and community groups, Wal-Mart had agreed to offer part-time employees, even those averaging less than 24 hours a week, health care insurance after a year on the job, shaving a year off the eligibility requirement. Wal-Mart also said that it was offering health plans that cost its employees about $250 a year for family coverage.

At the time, the moves were considered a departure from some of its major rivals and large employers, more than half of whom offer no company-sponsored health plan for part-time workers.

On Thursday, the company would not say what percentage of its work force was part time or worked fewer than 24 hours a week. Greg Rossiter, a Wal-Mart spokesman, said the decision to deny coverage to new part-time employees resulted from the company’s revamping of its health care offerings in light of rising costs.

“Over the last few years, we’ve all seen our health care rates increase and it’s probably not a surprise that this year will be no different,” Mr. Rossiter said. “We made the difficult decision to raise rates that will affect our associates’ medical costs. The decisions made were not easy, but they strike a balance between managing costs and providing quality care and coverage.”

The company said the changes were not a result of the new federal health care law. But the higher rates along with steep spikes in premiums for other plans this year are likely to stoke the national debate over the year-old legislation that has pitted President Obama and Democrats against Republicans opposed to the changes. Challenges to the law by several states are now before the Supreme Court.