Indianapolis ? Nearly one of every 10 midsized or big employers expects to stop offering health coverage to workers after insurance exchanges begin operating in 2014 as part of President Barack Obama’s health care overhaul, according to a survey by a major benefits consultant.

Towers Watson also found in its July survey that another one in five companies are unsure about what they will do after 2014. Another big benefits consultant, Mercer, found in a June survey of large and smaller employers that 8 percent are either “likely” or “very likely” to end health benefits after the exchanges start.

The surveys, which involved more than 1,200 companies, suggest that some businesses feel they will be better off dropping health insurance coverage once the exchanges start, even though they could face fines and tax headaches. The percentage of companies that are already saying they expect to do this surprised some experts, and if they follow through, it could start a trend that chips away at employer-sponsored health coverage, a long-standing pillar of the nation’s health system.

“If one employer does it, others likely will follow,” said Paul Fronstin of the Employee Benefit Research Institute. “You would see this playing out over the course of years, not months.”

A large majority of employers in both studies said they expect to continue offering benefits after these exchanges start. But former insurance executive Bob Laszewski said he was surprised that as many as 8 or 9 percent of companies already expect to drop coverage a couple of years before the exchanges start.

Such a move could lead to more taxes for both companies and employees, since health benefits currently are not taxed, and companies could be fined for dropping coverage. It also would give their employees a steep compensation cut if they don’t receive a pay raise, too.

“Dropping coverage is going to be very difficult for these (companies) to do,” said Laszewski, a consultant.

Towers Watson’s Randall Abbott said the survey results should be seen as a snapshot of how companies are thinking now, not as a final decision, because there still are many unresolved variables. Companies may change their thinking once they learn more about how the exchanges will work or whether employees will accept them.

The health care overhaul also faces court challenges, and President Obama is up for re-election next year, two more variables that could shape what happens in 2014 and afterward.

The Obama administration took issue with the Towers Watson survey, pointing out that studies by the nonpartisan Congressional Budget Office and nonprofits like Urban Institute reached different conclusions.

An Urban Institute study projected that the overhaul will have little effect on employer-sponsored insurance. When lawmakers debated the legislation, the CBO projected it would only have minimal impact on employer plans. About 3 million fewer people would be covered through work, but they’d be able to get insurance elsewhere.

Health and Human Services spokesman Richard Sorian said the administration expects to see a rise in employer-sponsored health insurance, not a decline.

“History has shown that reform motivates more businesses to offer insurance,” said Sorian. “Health reform in Massachusetts uses a similar structure, and the number of people with employer-sponsored insurance in Massachusetts has increased.”