July 18, 2017 12:54 PM EDT

Senate Republicans’ fallback plan to repeal the Affordable Care Act and work on a replacement later would cause 18 million more people to be uninsured and premiums to spike by 20 to 25 percent next year, according to a congressional report.

When Republicans passed a similar repeal-only bill in 2015, which President Obama vetoed, the Congressional Budget Office found that it would cause serious problems in the insurance market.

After several Republican senators came out against the Better Care Reconciliation Act, President Trump and Senate Majority Leader Mitch McConnell floated repealing the law first as a Plan B. But that plan has already faced criticism from other Republican senators, in part because of the effect founds by the old CBO report.

“Repeal and delay would create massive uncertainty,” said Bruce Siegel, president of America’s Essential Hospitals, an association of about 300 hospitals dedicated to providing health care to poorer Americans. “You would push the insurance marketplace into a death spiral.”

In a statement, McConnell said that “in the coming days” the Senate would vote on the 2015 bill as an amendment to the AHCA, the health repeal legislation passed by the House in May.

But Gary Claxton, vice president of the Kaiser Family Foundation, says that if the repeal-now, replace-later legislation is passed, it would cause the individual insurance marketplace to “meltdown.”

“In two years it would cause very significant problems in the non-group market,” he added. “The premiums would escalate very rapidly.”

Write to Jack Brewster at jack.brewster@time.com.

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