SPRINGFIELD — Concerns about the shaky state budget, a new union labor contract and threats to alter state retirement plans have triggered a major exodus of state employees.

The State Employees Retirement System, which administers pension benefits to state government workers ranging from prison guards to child welfare workers, is projecting a more than 40 percent spike in the number of people retiring during the fiscal year ending June 30.

The system’s executive secretary, Tim Blair, said the state is on track to see more than 4,000 workers join the ranks of retirees this year, compared with about 2,750 last year.

“We are definitely seeing an uptick,” Blair said. “There hasn’t been any slow-down.”

The Department of Corrections has already seen more than 680 workers retire this fiscal year. The Department of Human Services has the next highest number of retirees at 585.

Gov. Pat Quinn’s spending proposal for the fiscal year beginning July 1 allocates money for about 51,500 workers, down from about 54,500 when he took office in 2009. It remains unclear whether the spike in retirements will drive that number even lower.

Kelly Kraft, spokeswoman for Quinn’s budget office, said some jobs performed by the prospective retirees are too important to be left unfilled.

“Some of them, I’m sure, would be replaced,” Kraft said Thursday.

The potential decrease comes as other states are rebuilding their work forces in the wake of the recession. According to forecasts by Deutsche Bank, IHS Global Insight and Moody’s Analytics, states are projecting an overall increase in work force numbers in the next fiscal year.

A number of factors explain why more people than normal are poised to leave Illinois’ payroll.

Not only is the Baby Boom generation hitting retirement age, many of the state’s older workers are worried about what state officials will do to their pensions and their union contracts in the fiscal year beginning July 1.

For example, lawmakers and Gov. Pat Quinn are discussing a plan to revamp underfunded state pension systems that could include changes to benefits. Quinn is expected to announce the outline of a plan in the coming days. Older workers are hoping to lock in their current benefits to avoid having those changes affect them.

In addition, because of the state’s budget woes, many observers believe Quinn is arguing against awarding pay raises in the new labor contract, which is being negotiated behind closed doors. The current agreement expires June 30.

The governor also wants to close dozens of state facilities, affecting thousands of state workers, as part of his effort to balance the state budget.

A spokesman for the state’s largest employee union said the “constant torrent of public criticism” of government workers has pushed many of them toward the retirement line.

“I think the biggest factor is the sense that state employees are demonized, vilified and under attack,” said Anders Lindall, spokesman for the American Federation of State, County and Municipal Employees Council 31. “Public service in this country used to be considered a higher calling.“

Kraft said officials look at five-year retirement projections rather than at individual yearly numbers when they are planning the budget.

She said the budget office has not concluded that changes to pensions and the union contract are the reasons for the rise in retirements.

“We don’t necessarily see that as a trend, but that could be someone’s individual concern,” Kraft said.