About 100 Foreign Service officers, some in their mid-40s, may find their diplomatic careers abruptly halted next year as they are forced into early retirement because there aren't enough promotions to go around.

"These are good, solid performers and it's coming as something of a shock to them," said Dennis K. Hays, president of the American Foreign Service Association, the union of career diplomats.

The problem is the Foreign Service's new retirement system; it was designed to make way for up-and-coming employes by reducing the number of senior officers in a system that had been criticized as top-heavy.

Under the plan, if a career diplomat does not win a promotion into the senior ranks within six years, he or she is retired mandatorily. The diplomat decides when to begin the six-year clock, once he or she has reached the top of the middle ranks.

About 150 officers began the process in 1981, the first year the system went into effect, based on educated assumptions about what the promotion rate would be in the coming six years.

But promotion rates have been 30 percent to 40 percent lower since then because of the large number of diplomatic posts that have gone to political appointees, less voluntary attrition than anticipated and an increase, from 60 to 65, of the mandatory retirement age.

Most officers vulnerable to mandatory retirement are coming to the end of their six-year window in 1986. Those who began the process after 1981 "saw what was going on and they were timing themselves a little better," Hays said.

Hays said the union is working with management and affected officers to "come up with a system whereby we can preserve the basic concept of the act, which is that we want a lean -- not too lean -- service that is appropriately graded and ranked, and yet realize that these people still have a lot to contribute."