That grim analysis by the American Academy of Actuaries—composed of people who play a key role in setting insurance prices—also warned that Republican plans could lead to average costs rising for coverage for remaining customers, and "threaten insurer solvency" if insurers are temporarily barred from raising their prices, as some proposals suggest.
The actuaries' report comes as the Supreme Court is on the verge of deciding whether to effectively bar financial aid to Obamacare customers in 34 states served by HealthCare.gov, the federal insurance marketplace.
In anticipation of such a ruling, a number of congressional Republicans have floated various temporary fixes that would seek to protect at least existing customers from losing their subsidies. The proposals would keep that aid in place temporarily for varying lengths of time.
Read MoreObamacare case is political pickle for Republicans
But "most of the scenarios on the table leave open real risks for health plans in the market and make it likely that some plans will drop out while others will need to dramatically increase prices," said Caroline Pearson, senior vice president of the consulting company Avalere Health.
The risks are tied to the crucial role that federal subsidies play in Obamacare. The subsidies, which offset monthly premium prices, are available to customers who earn between one and four times the federal poverty level, or $11,670 to $46,680 for a single person.
Nearly 90 percent of the 11.4 million customers of all Obamacare exchanges in 2015 qualified for subsidies. And more than 9 million of those people are enrolled through HealthCare.gov.
It is widely accepted that many, if not most, of the subsidized Obamacare customers would drop their insurance if the aid was removed, either because they would then be unable to afford their plans or because they would opt not to buy them at the full price.
In the pending Supreme Court case, King v. Burwell, plaintiffs claim that the Affordable Care Act only authorizes subsidies for customers of an exchange established by a state, not one created by the federal government. That argument is based on the precise wording of the ACA, which does not discuss explicitly discuss the issuance of financial aid to HealthCare.gov customers, although it does do so for state-run exchange customers.
The Obama administration disputes that claim, arguing that the ACA allows subsidies for customers of all government-established exchanges, be they federal- or state-run.
If the subsidies are taken away for HealthCare.gov, an estimated 8.2 million people, primarily in Republican-led states, would lose their insurance, analysts predict. It would also lead to dramatically higher insurance premium prices as insurers deal with the costs of providing benefits to the remaining people in their plans, who would as a group would tend to be less healthy.
In response to that chance and the political risk it poses to them, several Republicans, despite being opposed to Obamacare, have suggested effectively continuing the subsidies to as late at the summer of 2017.