WASHINGTON - The Obama administration had a message Monday for employers who want to keep federal bureaucrats from rewriting the rules for their company medical plans: Don't jack up costs for workers, and you won't have to worry about interference from the new health care law.

"What we don't want is a massive shift of costs to employees," said Health and Human Services Secretary Kathleen Sebelius.

She announced a new regulation that spells out how health plans that predate the health overhaul law can avoid its full effect. Meant to deliver on President Obama's promise that people who like their current health coverage can keep it, the rule sets limits likely to become increasingly important as medical costs keep rising.

Plan changes that would cause a health plan to lose its "grandfathered" status and trigger new federal requirements include:

Dropping coverage for a particular health problem, for example, diabetes.

Increasing the proportion of insurance paid by workers, for example from 20 percent of the hospital bill to 25 percent.

Cutting back the share of premiums that the company pays by more than 5 percent.

Significantly increasing annual deductibles or co-payments paid by workers - for example, an employer raising a $1,000 deductible by $500 over the next two years.

Consumer advocates said the regulation gives employers the flexibility to make needed changes, while protecting workers.

"If a plan changes in some significant way, or if it increases cost-sharing amounts, then that results in a very different plan - and it should not be grandfathered in," said Ron Pollack, executive director of Families USA, an advocacy group that supports the overhaul law.

Employers were wary.

"It's a big unknown," said Steve Wojcik, vice president of the National Business Group on Health, which represents human resources managers at major companies. "It definitely sets boundaries where plans have been used to considering all kinds of changes to both improve quality and control costs."