For the first time in American history, a majority of union members are government workers rather than private-sector employees, the Bureau of Labor Statistics announced on Friday.

In its annual report on union membership, the bureau undercut the longstanding notion that union members are overwhelmingly blue-collar factory workers. It found that membership fell so fast in the private sector in 2009 that the 7.9 million unionized public-sector workers easily outnumbered those in the private sector, where labor’s ranks shrank to 7.4 million, from 8.2 million in 2008.

“There has been steady growth among union members in the public sector, but I’m a little bit shocked to see that the lines have actually crossed,” said Randel K. Johnson, senior vice president for labor at the United States Chamber of Commerce.

According to the labor bureau, 7.2 percent of private-sector workers were union members last year, down from 7.6 percent the previous year. That, labor historians said, was the lowest percentage of private-sector workers in unions since 1900.