WASHINGTON — Millions of older Americans who rely on federal benefits will get a 1.7 percent increase in their monthly payments next year, the government said today.

It’s the third year in a row the increase will be less than 2 percent.

The annual cost-of-living adjustment, or COLA, affects payments to more than 70 million Social Security recipients, disabled veterans and federal retirees. That’s more than a fifth of the country.

The increase amounts to about $20 a month for the typical Social Security recipient.

“The COLA helps beneficiaries of all ages maintain their standard of living, keeping many from falling into poverty by providing partial protection against inflation,” said Jo Ann Jenkins, who heads AARP.

By law, the increase is based on inflation, which is well below historical averages so far this year. Congress enacted automatic increases for beneficiaries in 1975, when inflation was high and there was a lot of pressure to raise benefits.

For the first 35 years, the COLA was less than 2 percent only three times. Next year, the COLA will be less than 2 percent for the fifth time in six years. This year’s increase was 1.5 percent, the year before it was 1.7 percent.

Social Security is financed by a 12.4 percent payroll tax on the first $117,000 of a worker’s wages — half is paid by the worker and half is paid by the employer. Next year, the wage cap will increase to $118,500.

About 59 million retirees, disabled workers, spouses and children get Social Security benefits. The average monthly payment is $1,192.

The COLA also affects benefits for about 4 million disabled veterans, 2.5 million federal retirees and their survivors, and more than 8 million people who get Supplemental Security Income, the disability program for the poor.

Advocates for seniors say the government’s measure of inflation doesn’t accurately reflect price increases because they tend to spend more of their income on health care.

The rise in medical costs has slowed in recent years, but people hit with serious illnesses can still see costs soar.