A new federal retirement plan -- to be introduced formally Tuesday -- will not change benefits or the retirement age for any of the 2.3 million U.S. employes under the current civil service pension system.

These employes, unless they choose to join the new system, will continue to get the same level of benefits and be able to retire at age 55 (after 30 years' service) without penalty. The pension program being proposed by Sens. William V. Roth (R-Del.) and Ted Stevens (R-Alaska) would be mandatory only for the 300,000 federal workers hired since January 1984.

Congress has until the end of this year to set up a new retirement program for the "new" hires. If it doesn't come up with a supplemental program for them, the employes -- who now pay the full Social Security tax plus a token contribution to the civil service retirement fund -- will have to put more than 14 percent of their salaries into the two systems starting next January.

The Roth-Stevens bill would:

*Set up a tax-deferred savings program that is more generous than most plans already available to private sector workers. Under the bill employes could voluntarily put up to 10 percent of their annual salary into a savings/investment program. The government would contribute another 5 percent to each employe's account. Those contributions would be tax-free until the worker retires and starts drawing the money or takes it out of the system.

*Have the government make all contributions to the civil service retirement program. Most federal workers now contribute 7 percent of their salaries to the plan, with the government contributing between 18 percent and 23 percent (depending on whose figures you use). Require employes under the new system to work until age 62 to get full benefits. Those who elect to retire at age 55 with 30 years' service would have their annuity reduced 2 percent for each year they are under age 62. Persons retiring at age 55 with less than 30 years' service would take a 5 percent reduction for each year under 30.

As the bill moves through Congress, U.S. employe unions will ask that basic benefit levels be improved. They are concerned that the important third tier of benefits -- the tax-deferred savings plan -- would be out of financial reach for many middle- and low-income civil servants.