WASHINGTON (Reuters) - U.S. private employers hired the fewest number of workers in 3-1/2-years in August and data for the prior month was revised lower, potentially hinting at a sharp labor market slowdown.

Private payrolls increased by 99,000 jobs this month, the smallest gain since January 2021, after rising by a downwardly revised 111,000 in July, the ADP National Employment Report showed on Thursday. Economists polled by Reuters had forecast private employment would advance by 145,000 positions after a previously reported gain of 122,000.

The ADP report, jointly developed with the Stanford Digital Economy Lab, was published ahead of Friday's more comprehensive and closely watched employment report for August from the Labor Department's Bureau of Labor Statistics. Initial prints have mostly understated private payroll growth this year.

Private payrolls likely increased by 139,000 jobs in August after rising by 97,000 in July, according to a Reuters survey of economists. With solid gains in government employment expected, nonfarm payrolls are forecast to have increased by 160,000 last month after advancing by 114,000 in July.

The unemployment rate is forecast to slip to 4.2% from nearly a three-year high of 4.3% in July.