President Barack Obama's health care overhaul is expected to hit nearly 6 million Americans with a tax penalty, most in the middle class.
The prediction from the nonpartisan Congressional Budget Office is much higher than expected. An earlier estimate projected 4 million people would be affected by the mandate, which goes into effect in 2016.
Anyone without health insurance at that time will be charged. The average penalty is $1,200.
Republicans are still working to repeal the health care law, even after the Supreme Court said the insurance mandate is constitutional.
"The bad news and broken promises from Obamacare just keep piling up," said Rep. Dave Camp, R-Mich., chairman of the House Ways and Means Committee.
A spokeswoman for the Obama administration claimed 98 percent of Americans won't be affected by the penalty, including those who can't afford health care.
"The individual responsibility policy will only affect people who can afford health care but choose not to buy it," said Erin Shields Britt, with the Health and Human Services Department.
Republican presidential candidate Mitt Romney has said insurance mandates should be up to each state.