BOSTON, May 17 (Reuters) - Chief executive officers of S&P 500 companies on average made 335 times more money than the average rank-and-file worker last year, down from a multiple of 373 in 2014, according to a union study released on Tuesday.

The figures released annually by the AFL-CIO, the largest U.S. federation of labor unions, are likely to gain attention. Pay disparities, which have persisted despite a steady American economy that has reduced the joblessness rate to around 5 percent and raised wages somewhat, have fueled political debate.

The average production and non-supervisory worker made around $36,900 last year, up from roughly $36,000 in 2014, according to a statement from the AFL-CIO.

Meanwhile the average CEO of an S&P 500 company made $12.4 million last year, down from $13.5 million in 2014. An AFL-CIO spokeswoman said the lower average CEO compensation figure reflected how for many the present value of future pension benefits declined.

Union leaders said the figures showed how pay decisions do not favor the average worker. "The income inequality that exists in this country is a disgrace," AFL-CIO President Richard Trumka said in a statement. "We must stop Wall Street CEOs from continuing to profit on the backs of working people."

The high levels of executive pay have drawn criticism from both Democrat Hillary Clinton and Republican Donald Trump in the current U.S. presidential campaign.

Nonetheless, top shareholders have overwhelmingly supported management on executive compensation decisions, according to the advisory "say on pay" votes most public companies hold annually.

Starting in 2017, the U.S. Securities and Exchange Commission will require public companies to disclose the ratio of the pay of their CEO to the median compensation of their employees.

(Reporting by Ross Kerber; Editing by Lisa Von Ahn)