Health care costs at U.S. group health plans are on track to rise an average of 10.5% over the next 12 months, a consulting firm says.

The consulting arm of Aon Corp., Chicago, says the projected rate of increase for the most common form of group coverage, a preferred provider organization plan, will increase about 10.7% for employers that buy health coverage together with prescription drug coverage.

The 2008 rate of increase for PPO plans with drug coverage was also 10.7%.

Aon consultants have based the projections on results from a survey of large health insurers.

The projected 2009 increase for the second most popular type of plan — a high-deductible health plan that includes prescription drug coverage and access to a health savings account or a health reimbursement arrangement — is 10.5%. Like the projected PPO rate of increase, the projected rate of increase for health account plans is the same as in 2008.

The only signficant change in projected health care cost trends affects employers that still provide retiree health benefits. The projected rate of increase for retiree benefits has dropped to 6.6%, from 7.3%, with or without prescription benefits, Aon reports.