The percentage of the largest U.S. employers that offer a defined benefit pension plan to new salaried employees continues to fall, according to a new report.

As of June 30, 30% of Fortune 100 companies offered a defined benefit plan to new salaried employees, according to New York-based Towers Watson & Co. That's down from 32% at the end of 2012, 37% in 2010 and 43% in 2009.

As recently as 1998, defined benefit plans were the norm among the nation's largest employers, when 90% of Fortune 100 companies offered the plans to new salaried employees.

Since then, large employers have moved away from the plans. “The shift is motivated by several factors, including employers' desire to reduce overall retirement costs — perhaps due to higher compensation and benefit costs elsewhere, especially health care — perceptions that workers prefer account-based plans, market trends, and a desire to reduce financial risk,” Towers Watson said in an article published last week in The Insider, a company publication.

The move away from defined benefit plans has been especially pronounced for traditional plans, in which the benefit is typically based on employees' years of service and employees' salary during their last years of employment.

Just seven Fortune 100 companies offered a traditional defined benefit plan to new salaried employees as of June 30, down from nine in 2012, 14 in 2011 and 17 in 2010.

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By contrast, during the 1980s, defined benefit plans were the norm among Fortune 100 companies. In 1985, for example, nearly 90% of Fortune 100 companies offered a traditional defined benefit plan to new employees.

The prevalence of hybrid plans, typically cash balance plans, also has sharply declined. As of June 30, 23 Fortune 100 companies offered hybrid plans to new salaried employees. That's unchanged from 2012, but a steep decline compared with 2004, when 35 Fortune 100 companies offered the plans. While hybrid plans have defined benefit and defined contribution plan elements, legally they are defined benefit plans.

While a handful of big employers, including Dow Chemical Co. and The Coca-Cola Co., have set up new cash balance plans in recent years, new formations have been more than offset by other Fortune 100 companies, including Bank of America Corp., SunTrust Banks Inc. and Wells Fargo & Co., which began to phase out their cash balance plans.

As employers have moved away from defined benefit plans, the overwhelming majority of Fortune 100 companies now offer only a defined contribution plan to new salaried employees, according to Towers Watson.

As of June 30, 70% of the Fortune 100 offered only defined contribution plans, up from 68% in 2012, 67% in 2011 and 63% in 2010. By contrast, as recently as 1998, just 10% of Fortune 100 companies offered only defined contribution plans.

As employers have shifted to an all-defined-contribution-plan approach, more companies have taken steps to increase the likelihood that employees will save for retirement.

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For example, an earlier Towers Watson survey found that in 2012, 65% offered an automatic enrollment feature, up from 57% in 2011 and 51% in 2010. Under automatic enrollment, employees who don't respond to participation notices are enrolled automatically in a plan unless they notify their employers by a set date that they want to opt out.

In addition, more than 70% of employers offering automatic enrollment include an automatic escalation feature in which contributions automatically increase by a certain percent or amount each year, Towers Watson found.