U.S. debt held by the public will balloon to 150 percent of economic output by 2047 unless tax and spending laws are changed, the Congressional Budget Office said on Thursday, far exceeding the record level just after World War II.

The new projections show steeper 30-year debt growth than last year's long-term forecast by the non-partisan budget analysis agency, and could make it harder for some members of Congress to support a tax reform plan that is partly financed with higher deficits.

Last year, the CBO estimated U.S. public debt would grow to 141 percent of gross domestic product by 2046, while the record was 106 percent of GDP in 1946. That level would be reached in 2035, the CBO said.

Under this year's projection, CBO predicts public debt for 2017 to be about 77 percent of GDP, growing to 89 percent in 2027, and 113 percent in 2037.

The new forecasts assume that the Affordable Care Act, the healthcare law known as Obamacare that House Republicans failed to replace last week, stays in place for the long term.

The projected debt growth reflects CBO's estimates of the rising costs of caring for a growing population of people over 65, growth in interest costs, and assumptions of slower economic growth due to reduced assumptions about productivity gains.

Annual deficits are expected to average 8.6 percent of GDP in the 2038-2047 period versus 2.9 percent expected for 2017 and 4.0 percent for the 2018-2027 period.

The long-term projections extend CBO's assumptions made in its 10-year budget outlook in January, which showed falling deficits for the next two years, but growth thereafter.

Net interest costs in 2047 are expected to be 6.2 percent of GDP, compared with 1.4 percent in 2017 and about 1.2 percent in 1967, the CBO said.

But the wild card in the CBO 30-year projections are real interest rates. Real interest yields on 10-year Treasury notes are assumed to average 1.5 percent over 30 years, rising to 2.3 percent in 2047.

Much higher interest rates, coupled with different assumptions in productivity and healthcare cost growth, could produce significantly different outcomes, CBO said.

The CBO said that its debt-to-GDP ratio could range from 85 percent with low rates and strong growth, to 244 percent with weaker growth and higher rates.