Threats by President Donald Trump to ax crucial payments to health insurers will double, on average, the price of most Obamacare plans in California next year.

Obamacare officials in the Golden State on Wednesday said they are slapping a 12.4 percent surcharge on the prices of so-called silver individual health plans in 2018 because of uncertainty over whether Trump will continue making those payments.

And it will be taxpayers, not the insurance customers themselves, who will end up footing much of the bill.

Without the surcharge, silver plans sold on Covered California, the state's Obamacare marketplace, would have risen an average of just 12.5 percent next year, officials said as they revealed overall prices statewide.

With the surcharge, the average price hike is just shy of 25 percent.

For individual insurers, the actual added surcharge per plan will be anywhere from 8 percent to 27 percent.

About 6 out of 10 customers of Covered California are enrolled in silver plans, which cover 70 percent of enrollees' health costs.

Covered California said that even with the surcharge, 78 percent of customers who qualify for federal subsidies to reduce their monthly premiums will "either see no change in what they would pay for insurance in 2018, or would pay less than what they would have paid if there had been no ... surcharge."

That is because the value of those premium subsidies, which are in the form of federal tax credits, rises with the retail price of the health plan. In "many" cases, Covered California said, subsidized silver plan customers will see their subsidies rise more than the amount of the surcharge.