People buying individual healthcare policies would face sharply higher premiums, and some may be left with no insurance options if President Donald Trump makes good on his threat to stop "Obamacare" payments to insurers, congressional experts say.
The nonpartisan Congressional Budget Office also estimated that cutting off the payments would add $US194 billion ($A248 billion) to federal deficits over a decade. That's because other Affordable Care Act subsidies would automatically increase as premiums rise, more than wiping out any savings.
Sticker-price premiums for standard plans would rise about 20 per cent before factoring in federal tax credits for consumers, the CBO said on Tuesday.
About 1 million people would become uninsured right away, but within a few years that slippage would reverse and more people would be covered, the budget office added.
At issue are so-called "cost-sharing" payments, totalling about $US7 billion this year, that reimburse insurers for subsidising out-of-pocket costs for people with modest incomes.
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Trump has threatened to cut off the monthly payments, most recently after the collapse of the GOP healthcare bill.
The subsidies can cut a deductible of $US3500 down to a few hundred dollars. Nearly three in five HealthCare.gov customers qualify for cost-sharing help, an estimated six million people or more.
But the money is under a legal cloud because of a dispute over whether the Obama-era law properly authorised the monthly payments.
For months, Trump has been raising the prospect of terminating payments as a way to trigger a crisis and get Democrats to negotiate on a health care bill.
After the GOP drive to repeal "Obamacare" collapsed, the president tweeted: "As I said from the beginning, let ObamaCare implode, then deal. Watch!"
Trump elaborated in another tweet, "If a new HealthCare Bill is not approved quickly, BAILOUTS for Insurance Companies...will end very soon!"
Insurer groups say there's been no signal that the administration will stop making payments expected this month.
Leading Republican lawmakers have called for continuing the payments, at least temporarily, to ensure market stability.
