By Daniel Wiessner

The agency that enforces U.S. labor laws on Wednesday made it more difficult for businesses to defend workplace rules that could interfere with employees' rights to join unions, as part of a case involving waste disposal company Stericycle.

The Democrat-led National Labor Relations Board's 3-1 decision could provide a boost to unions amid an uptick in labor organizing that includes high-profile campaigns among employees of Starbucks Corp, Amazon.com Inc, Apple Inc and other large companies.

Routine work rules like those banning disparagement, solicitation or inappropriate social media posts are illegal if a worker could reasonably construe them as prohibiting protected activity such as discussing working conditions or distributing union literature, the NLRB said.

The board overturned a 2017 ruling by appointees of Republican former President Donald Trump that said workplace policies were valid as long as they were adopted for legitimate business reasons.

NLRB Chair Lauren McFerran in a statement said the Trump-era decision, which involved Boeing Co, did not give enough consideration to the chilling effect that workplace policies can have on workers' labor rights.

"Under the new standard, the Board will carefully consider both the potential impact of work rules on employees and the interests that employers articulate in support of their rules," said McFerran, an appointee of Democratic President Joe Biden.

Wednesday's ruling stems from a union's challenge to Stericycle Inc's policies barring worker conduct that could harm the company's reputation or create conflicts of interest.

The labor board did not rule on the merits of the case but sent it back to an administrative judge to apply the new standard.

The board's Democratic majority said employers can rebut the presumption that a rule is unlawful by proving to the NLRB that it "advances a legitimate and substantial interest" that cannot be addressed with a narrower policy.

NLRB Member Marvin Kaplan, a Trump appointee, in a dissenting opinion said that standard will be virtually impossible for many employers to meet. He added that his colleagues had failed to balance workers' labor rights with businesses' ability to adopt reasonable rules.

(Reporting by Daniel Wiessner in Albany, New York; editing by Susan Heavey)