The new health law is projected to reduce the total number of hours Americans work by the equivalent of 2.3 million full-time jobs in 2021, a bigger impact on the workforce than previously expected, according to a nonpartisan congressional report.

The analysis, by the Congressional Budget Office, says a key factor is people scaling back how much they work and instead getting health coverage through the Affordable Care Act. The agency had earlier forecast the labor-force impact would be the equivalent of 800,000 workers in...