The share of eligible workers without high school diplomas in the American workforce is climbing faster than the more educated groups that make up the bulk of the domestic jobs market, elevating the labor participation of "prime age" workers to their highest levels in decades.

Some 47.6% of workers in America without a high-school diploma were working or looking for work in September, up from 45.9% the same month five years ago, according to the latest US Bureau of Labor Statistics data. That educational group is the only one within the pool of workers ages 25-54 that saw an increase over that time, as more educated workers have not yet rejoined the workforce at their pre-COVID rates.

More broadly, the prime-age labor participation rate in August and September hit its highest levels since early 2001 at 84% and 83.8%, respectively. The growth in less-educated workers entering the labor market reflects a growing number of immigrants in the jobs market, an ongoing push to fill many low-paying jobs in the services sector, stubbornly high inflation driving many lower income Americans back into the workforce and declining high school drop-out rates effectively shrinking the size of the group, economists said.

"Ample opportunities, combined with strong wage incentives, pulled these workers into the labor force," said Oren Klachkin, a financial market economist at Nationwide. "Also, the lower income cohorts drew their excess savings down the quickest, so likely had little choice but to return to work."

The labor participation rate is a measure of the share of working age Americans either working or looking for work. The overall rate across the entire US workforce was 62.7% in September, still below the pre-pandemic rate of 63.2% in September 2019.

Workers with a bachelor's degree or higher had the highest participation rate in September at 72.9%, but that is down from 73.4% a year ago and the pre-pandemic rate of 73.9% in September 2019.

Americans with some college or an associate degree and high school graduates with no college saw similar declines. The participation rate for those with some college was 63.1% in September, down from 64.9% in September 2019 and the rate for those with only a high school diploma was at 56.7%, down from 58.1% five years earlier.

The recent sharp rise in immigration is likely boosting the rate of the lowest-educated workers participating in the labor force because fewer immigrants have degrees or high school diplomas. And while there may not be more unskilled jobs than there were prior to the pandemic, there could be some short-term increases due to the trend against globalization, said Thomas Simons, a senior economist at Jefferies.

There is not a clear change drawing more Americans without high school degrees into the workforce, such as the loss of some sort of government assistance, but inflation could be one explanation as rising prices tend to negatively impact lower income-families more than wealthier ones, Simons said.

The divergence is also taking place in a much tighter labor market than 2019, said Cory Stahle, an economist for Indeed Hiring Lab. There are roughly 13% more job postings now than there were at the beginning of 2020, according to the Indeed Job Postings Index.

Though the labor market has cooled over the last two years, much of that has been driven by a decline in postings for "knowledge-working industries," which often require at least a college degree, Stahle said. Skilled labor jobs in construction, production and manufacturing have remained steady.

"This has led to more opportunities for job seekers in those sectors and stronger-than-average wage growth," Stahle said. "Wages in low-paying sectors grew fastest in the years following the pandemic, so it is likely that higher-paying jobs are luring more workers into the market."

While the participation rate for those without a diploma has increased, so has the unemployment rate for those individuals, which was at 6.8% in September, up from 4.9% in September 2019.

"Since being unemployed counts as participating in the labor force, part of the story here is that more people are entering or re-entering the labor force to look for work, but remain unemployed," Stahle said.

The overall number of workers without diplomas, however, is in decline. There were nearly 64 million workers in the American labor force with bachelor's degrees in September, up from nearly 57.9 million in September 2019. Workers without high school diplomas is dropped to 8.6 million in September, from more than 9.4 million in September 2019. The participation rate for this group, however, grew 170 basis points over the same period.

Workers without high school diplomas comprise the smallest group by educational attainment, leading to regular data fluctuations.

"There is a clear rising trend in participation for this group in recent years, but there is also a lot of noise," Stahle said.