Longtime federal workers could keep their current retirement package or switch over to a new pension plan offering generous tax breaks and investment options under a last-minute congressional compromise being studied by the administration.

The new system, mandatory for all workers hired since January 1984, will go into effect in June if the White House approves.

Retirement benefits would be based on Social Security, a modified civil service pension and earnings from optional investments employes could make.

The new system would permit workers to put up to 10 percent of their salaries each year into a tax-deferred savings/investment fund, and the government would match a portion of each employe's contribution. The system would be slightly more generous than the 401(k) programs available to many private-sector employes.

Congress is considering cutting back or eliminating 401(k) plans as part of tax reform. If those changes were made for private-sector programs, they also would apply to any system set up for government employes.

If the administration rejects the compromise or demands significant changes, congressional sources say, the search for a supplemental plan for the 300,000 feds hired since January 1984 will be ended. In that case, they say, those employes would begin paying for double coverage -- Social Security and civil service retirement -- a week from today.

That would mean those employes would be making the 7.15 percent contribution to Social Security, plus at least 7 percent of their salaries to the civil service retirement program.

New hires currently pay the full Social Security tax, but pay only 1.3 percent toward civil service retirement. The effect of double coverage would mean an additional bite of at least 6.7 percent from the paychecks of post-1983 civil servants. There would be no change for employes hired before that date, who pay only the Medicare portion of Social Security.

The compromise, worked out by the Senate Governmental Affairs Committee and the House Post Office-Civil Service Committee, has two major flaws as far as the administration is concerned: cost and the early retirement features.

The administration opposes extending the early retirement provisions of the current civil service pension plan -- allowing retirement at age 55 after 30 years' service -- to new employes. It wants the cost of any new federal retirement program held below the current cost of the current retirement system, 25 percent of payroll. Meetings

Baltimore will host the Aug. 11-15 convention of the National Association of Postal and Federal Employees union. Scheduled to speak are Rep. Parren Mitchell (D-Md.), NAACP President Benjamin Hooks and Virginia Lt. Gov. L. Douglas Wilder. For details, call Shelia High King, 939-6325.

The Association of Government Accountants Montgomery-Prince George's chapter will hold its May 14 dinner meeting at the Ramada Inn in Lanham. Dr. Estelle Ramey of Georgetown University will speak. For reservations, call Frank Marshall, 755-4671.

The Federal Executive and Professional Association will hold its annual meeting Saturday morning at the National Institutes of Health in Rockville. For information, call 384-2616.

House Majority Leader Jim Wright (D-Tex.) will speak at the May 1 meeting of the House Legislative Assistants Association. The meeting is scheduled for 6 p.m. in the Education and Labor Committee Room in the Rayburn Building. For details, call Sylvia Binkowski, 225-6261.