WASHINGTON — Your medical plan is facing an unexpected expense, so you probably are, too. It’s a new, $63-per-head fee to cushion the cost of covering people with pre-existing conditions under President Barack Obama’s health care overhaul.

The charge, buried in a recent regulation, works out to tens of millions of dollars for the largest companies, employers say. Most of that is likely to be passed on to workers.

Employee benefits lawyer Chantel Sheaks calls it a “sleeper issue” with significant financial consequences, particularly for large employers.

“Especially at a time when we are facing economic uncertainty, (companies will) be hit with a multi-million dollar assessment without getting anything back for it,” said Sheaks, a principal at Buck Consultants, a Xerox subsidiary.

Based on figures provided in the regulation, employer and individual health plans covering an estimated 190 million Americans could owe the per-person fee.

The Obama administration says it is a temporary assessment levied for three years starting in 2014, designed to raise $25 billion. It starts at $63 and then declines.

Most of the money will go into a fund administered by the Health and Human Services Department. It will be used to cushion health insurance companies from the initial hard-to-predict costs of covering uninsured people with medical problems. Under the law, insurers will be forbidden from turning away the sick as of Jan. 1, 2014.

The program “is intended to help millions of Americans purchase affordable health insurance, reduce unreimbursed usage of hospital and other medical facilities by the uninsured and thereby lower medical expenses and premiums for all,” the Obama administration says in the regulation.