A historically high number of people will be locked out of the workforce by 2021, according to a report by the Congressional Budget Office released Tuesday.

President Barack Obama's signature health-care law will contribute to this phenomenon, the CBO said, citing new estimates that the Affordable Care Act will cause a larger-than-expected reduction in working hours—eliminating the equivalent of about 2.3 million workers in 2021.



In 2011, the CBO estimated the law would cause a reduction of about 800,000 full-time equivalent workers.

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"CBO estimates that the ACA will reduce the total number of hours worked, on net, by about 1.5 to 2 percent during the period from 2017 to 2024, almost entirely because workers will choose to supply less labor—given the new taxes and other incentives they will face and the financial benefits some will receive," said the report.



"The reduction in CBO's projections of hours worked represents a decline in the number of full-time-equivalent workers of about 2.0 million in 2017, rising to about 2.5 million in 2024," it added.

The White House responded to the report, saying it does not show that Obamacare will reduce jobs.



"CBO's findings are not driven by an assumption that ACA will lead employers to eliminate jobs or reduce hours, in fact, the report itself says that there is 'no compelling evidence that part-time employment has increased as a result of the ACA,'" said a statement from the White House.



The report also does not include analysis of the impact the ACA will have on labor productivity, senior administration officials told CNBC. They noted the law could in fact reduce employee absenteeism and improve output.

The CBO estimated last year that 7 million people would sign up for health insurance through Obamacare in 2014, but computer problems have now lowered their estimate by a million, The Wall Street Journal reported.



(Read more: Obamacare computers not yet equipped to fix errors: Report)