White-collar federal employees made about 26 percent less than their private sector counterparts last year, The Washington Post reported.
The pay gap is based on data from the Bureau of Labor Statistics that by law are supposed to be used in setting annual General Schedule pay raises.
"This clearly shows that there is a pay gap and that federal employees are underpaid," said J. David Cox, president of the American Federation of Government Employees. "Hopefully, we can get back to reasonable cost-of-living adjustments and work on the pay gap."
Republican presidential nominee Mitt Romney argues that federal workers are overcompensated by 30 percent to 40 percent on average.
"No one who takes a look at these numbers, other than federal employee unions, concludes that federal employees are massively underpaid," said James Sherk, the Heritage Foundation senior policy analyst who performed the think tank's study.
"I think on federal pay there's too much misinformation and fiction out there," said Colleen M. Kelley, president of the National Treasury Employees Union. "There's a very long history to this methodology. Even if someone wants to argue with what the number is, it's important to address that there is a gap and it continues to grow. There's no way to make the number zero, if you're basing it on facts."