Insurers who sell coverage on California's exchange will be increasing premiums by an average of 12.5 percent for 2018, state officials announced Tuesday.

That number, however, could almost double if the Trump administration chooses to cut off insurer payments known as cost-sharing reduction payments, said Peter Lee, the executive director of Covered California, the name of the state's exchange. Dropping the subsidies would cause premiums to rise by an additional 12.4 percent.

The funds, which are mired in a legal battle that began under the Obama administration and now are under the purview of the Trump administration, help insurance companies reduce out-of-pocket medical expenses for nearly 6 million low-income people who buy coverage on Obamacare's exchanges nationally.

President Trump has indicated that he is considering cutting the funds off, a move that would result in premium increases and insurer exits across the country, a trend that already has begun in some states but would become aggravated in others.

Officials from Covered California sent a letter on Tuesday to Health and Human Services Secretary Tom Price and Centers for Medicare and Medicaid Services Administrator Seema Verma saying that they needed the administration to make a decision about what would happen to the payments. Trump is expected to make a decision as early as Tuesday.

In a phone call with reporters, Lee said that "a clear and definitive policy guidance," would be necessary for certainty, adding that "a tweet would not be enough."

One option, he said, would be for Congress to appropriate the funds, expected to reach $7 billion across the country this year.

Other than the uncertainty about the funds, health officials in California said they believed the outlook was generally strong for Covered California, even as other states have released projections that show higher premium increases and as some states have counties facing the prospect of no insurer to buy coverage from next year.

In California, all 11 insurers who sold plans in the exchange last year will be applying in the exchange in 2018, and 83 percent of the 1.4 million people who buy plans will have three or more health insurers to choose plans from.

"While there is ongoing uncertainty and a lack of clarity at the federal level, consumers who need affordable health insurance will continue to have good choices in Covered California next year," Lee said in a statement.

One insurer, however, Anthem Blue Cross Blue shield, is withdrawing from 16 of California's 19 regions, meaning that the 153,000 people who receive coverage from Anthem will need to switch plans. The 108,000 customers in the leftover three counties who get Anthem coverage can either stay enrolled in the same plan or switch to another.

"The enrollees who are affected by Anthem's decision to pull out of some regions should know that their existing coverage will remain intact throughout the remainder of 2017, and they will have good options when they switch plans for 2018," Lee said in a statement.

State officials estimate that rate increases would have been even lower, at under 10 percent, if the health insurance tax were to be suspended again for next year, accounting for an increase of 2.8 percentage points. Members of Congress are exploring ways to work in a bipartisan manner on Obamacare and may choose to suspend the tax again as a way to lower premiums across the country.

About 6 to 7 percentage points were attributed to a rise in medical costs.

Most customers on the exchange, 85 percent, will not personally feel the premium increases, because federal subsidies will rise to offset the increase on customers. If customers switch to a plan in the same level they have currently, the lowest increase they would see is of 3.3 percent.