If Congress fails to authorize a new federal pension program or extend the existing one by the end of the month, several hundred thousand government workers -- virtually everyone hired by Uncle Sam since January 1984 -- will have their take-home pay reduced by about 5 percent as their contributions to civil service retirement increase next month.

The employes, who now have 8.35 percent taken from their salaries for Social Security and civil service retirement, would have their deductions raised next year to 14.15 percent if no action is taken.

They currently pay the full 7.05 percent Social Security tax and 1.3 percent for civil service benefits, under an interim arrangement that expires Dec. 31.

But if Congress fails to approve a new pension plan or extend the interim program into next year, those post-1983 hires will be required to make full contributions to both Social Security and the civil service retirement system.

The Social Security tax next year goes to 7.15 percent, and the new employes would have to contribute the full 7 percent for civil service benefits.

The Senate has already approved two new retirement options for post-1983 hires. The House Post Office-Civil Service Committee has cleared a plan of its own, but it is unlikely that final congressional action will take place this year.

Congressional insiders anticipate that at the last minute, the Senate and House will agree to extend the current Social Security-civil service contribution formula for two or three months.

Workers hired before 1984 will notice a slight drop in take-home pay because the Medicare deduction from their checks, now 1.35 percent on amounts up to $39,600 a year, goes up next month to 1.45 percent on amounts up to $42,000.

"We expect the interim arrangement will be extended as part of the third continuing resolution," a congressional aide said yesterday. Congress is expected to approve the resolution, which permits agencies without approved budgets to keep operating, before the current session ends.