About 88.1 percent of federal employees in the Federal Employment Retirement System contributed to their TSP accounts in February – higher than at any other point. In 2014, the FERS participation rate was 87.5 percent.

The growth in participation rates is partly due to auto-enrolling federal employees unless they specifically opt out, according to the agency. About 3.9 percent of participant accounts have opted out.

"We are very pleased that auto enrollment is having the desired effect of getting more FERS participants contributing to their retirement," said Kim Weaver, the director of external affairs at the FRTIB. "Contributing to the TSP is vital to allowing our participants to retire with dignity."

The participation rate among Civil Service Retirement System members stood at 59.6 percent while members of the military were at 41 percent.

The overall funds invested in the TSP also hit a record high - $451 billion in February – according to the agency. Invested funds have more than doubled since 2006, when the TSP held just more than $200 billion.

The average balance in a FERS account stands at $117,584 according to the agency.

Jessica Klement, legislative director at the National Active and Retired Federal Employees Association, called the latest numbers "fantastic" and said the increasing numbers is good news.

"This is one of the best-managed retirement plans in the entire world," Klement said. "We supported automatic enrollment because it's in the best interest of employees."