President Reagan's plan to raise the federal retirement age to 65 will contain a grandfather clause aimed at protecting the benefits of employes who are 55 or older at the time of enactment.

Next week the administration will propose legislation that would force federal employes (who can now retire at age 55 with 30 years service) to work another 10 years to get full benefits.

Currently, employes with 30 years of service who retire get an immediate annuity equal to just over 56 percent of their highest three-year average salary. People retiring earlier take a 2 percent annuity cut for each year they are under 55.

If Congress goes along with the proposed change, part of a larger "reform" of the federal retirement system, workers who become 55 after enactment would have to work 10 more years to get unreduced benefits. Those who retired before age 65 would take a 5 percent annuity reduction for each year they were under 65.

But the proposed legislation will allow workers who become 55 before enactment to remain under the current retirement system. That means they could retire at 55 or work a few more years and still be able to retire without the 5 percent penalty being applied to them.

The legislation will also propose that all employes put more of their salary into the retirement fund. They now contribute 7 percent. That would be raised to 9 percent in 1984, and to 11 percent the following year.