Group health care plan costs jumped an average of 6.9% in 2010, the biggest increase since 2004, according to a survey of more than 2,800 employers released Wednesday by Mercer L.L.C. in New York.

That 6.9% increase brought costs up to an average of $9,562 per employee compared with an average of $8,945 per employee in 2009, according to the survey.

By contrast, costs rose by an average of 5.5% in 2009, the lowest increase in more than a decade, while costs climbed an average of 6.3% in 2008. Between 2005 through 2007, costs climbed by an average of 6.1% in each of those three years.

Mercer consultants said the spike in costs may be the result of two factors: medical providers boosting their fees and charges, and an increase in utilization.

“Higher prices for health care services seem to be part of the equation, but if the recession caused a slowdown in utilization last year, we may also be seeing the effect of employees getting care they’ve been putting off,” Susan Connolly, a partner in Mercer’s Boston office, said in a statement accompanying the survey.

To prevent even bigger cost increases in 2011—caused in part by meeting requirements, such as extending coverage to employees’ adult children up to age 26, set by the health care reform law and which kick in next year—many employers intend to make plan design changes, such as shifting more costs to employees or changing insurers.

Without health plan changes, employers predicted cost increases of about 10% next year. With design and other changes, employers expect to hold down their actual cost increase to an average of 6.4% in 2011, according to the survey.

Employers already are taking action to try to hold down plan cost increases. For example, among preferred provider organization plans imposing a deductible, the average deductible for individual coverage through in-network providers jumped by more than $100 in 2010, rising to an average of $1,200.

Similarly, the percent of PPO sponsors that do not require a deductible for individual coverage from in-network providers fell to 16% in 2010, down from 22% in 2009.

In addition, more employers stopped offering health maintenance organizations—the most expensive plan design, with costs in 2010 averaging $8,892 per employee—while more employers added consumer-driven health care plans, such as plans linked to health savings accounts, with costs averaging $6,759 per employee.

In 2010, 26% of employers offered an HMO, down from 28% in 2008. As recently as 2005, just over one-third of employers offered an HMO.

On the other hand, this year, 17% of employers offered a CDHP linked to an HSA or health reimbursement arrangement, up from 15% in 2009. In 2005, just 2% of employers offered a CDHP.

The nation’s biggest employers have especially embraced CDHPs. This year, 51% of employers with at least 20,000 employees offered a CDHP linked to an HSA or HRA, up from 43% in 2009.

Among those jumbo employers, 15% of employees enrolled in CDHPs this year, up from 9% in 2009.

Copies of the “National Survey of Employer-Sponsored Health Plans” will be published in March. The report costs $600, and the report with tables costs $1,200. More information is available at www.Mercer.com/ushealthplansurvey or from Tara Lewis at 212-345-2451.