Many smaller health insurers may be forced to withdraw from the market as a result additional expenses created by the federal Patient Protection and Affordable Care Act, a study by Weiss Ratings predicts.

However, most large insurers have the capital and efficiencies of scale necessary to provide the expanded coverage mandated by the new law, the report issued Thursday by the Jupiter, Fla.-based rating agency said.

“Sweeping changes mandated by health reform, such as removal of certain limits and mandated coverage for pre-existing conditions, will inevitably force health insurers to spend more on medical care,” said Martin D. Weiss, president of Weiss Ratings, in a statement.

Most large insurers will be able to handle the extra costs, but “weaker, less profitable insurers” may be forced out of the market, reducing competition, he said.

“Already, even before any additional expenses mandated by health care reform, 174 health insurers reported losses last year,” Mr. Weiss said.

Of the 585 health insurers included in the Weiss Ratings study, 95, or 16.2%, received a rating of D+ (weak) or lower, putting them at risk of future financial difficulties caused by higher medical costs, a weaker economy or other pressures. In addition, another 186, or 31.8% of the insurers studied, are rated C+, C or C- (fair), signaling they also could have some difficulty absorbing the additional costs mandated by health reform, Weiss reported.