The steady rise in the cost of living has set the stage for a multimillion-dollar federal-military pension increase this fall, and has already triggered a 5.3 percent increase in payments for former government employes drawing disability payments.

Pensions of nondisability federal and military retirees will increase at least 3.9 percent next September, because of the continuing rise in living costs. Yesterday, the Labor Department released the Consumer Price Index for the month of May. It translates into a minimum 3.9 percent federal-military annuity increase effective Sept. 1 and payable in checks delivered to more than 100,000 retirees here in October.

Living costs for the month of June still must be tabulated before the government sets the exact amount of the multimillion-dollar pension increase it will make this fall to retirees. The June Consumer Price Index will not be made available until late in July. If the June CPI figure is higher than the May level, federal-military pensions will be increased accordingly.

Meantime, more than 40,000 former federal workers drawing compensation for serious work-related illness or injury are already guaranteed a 5.3 percent increase that is retroactive to May 1. Those former workers and their families will be getting the extra payments later this summer.

An estimated 46,000 former federal workers, or their survivors, are now receiving so-called FECA payments for job related injury or illness that prevents them from working elsewhere.

The tax-free disability pay is equal to almost 67 percent of salary for an individual, and up to 75 percent of full pay for disabled employes with dependents. Surviving spouses, dependent parents and children are also eligible to receive FECA payments based on the disability compensation of a federal or postal worker.

To be eligible for the 5.3 percent disability cost of living raise, an individual must have been on the disabled rolls for more than one year. Officials estimate that 40,000 of the 46,000 now getting payments will receive the added 5.3 percent.

The disability payment increases are made regionally, so recipients in one part of the country may get them sooner than people in another region. Labor Department officials believe most will get the extra 5.3 percent by August or September. A few may receive them in July. Whenever the COL is added to the checks, it will include retroactive payments to May 1 when the actual increase went into effect.

A very rough estimate is that the average tax-free disability payment under FECA is between $9,000 and $10,000 a year.

Regular retired federal and military personnel are also tied to the cost of living as measured by the Consumer Price Index. But they are under a different system which guarantees them two raises a year (March and September) so their increases are always for different amounts than for persons receiving disabled pay.

The exact amount of the September pension raise for federal-military retirees will not be known until about this time next month. That is when Consumer Price Index data for the month of June will be released.

The CPI for the month of April pushed the federal-military pension increase up to 2.8 percent. Yesterday's release of May CPI figures raised the amount of the cost-of-living raise to 3.9 percent. If, as expected, the CPI for the month of June goes up, the pension increase will go up accordingly.

For the moment, it stands at a guaranteed minimum of 3.9 percent.