NEW YORK (NewsNation) — Teamsters UPS drivers voted to authorize a strike Friday — a move that could bring the world’s largest package delivery service to a standstill, and significantly upend the U.S. supply chain this summer.

However, the authorization vote doesn’t automatically mean a strike will occur. Instead, it gives the International Brotherhood of Teamsters leaders the right to call for a national walkout if both sides fail to reach an agreement before their contract expires on July 31.

After the vote, UPS said the results “do not mean a strike is imminent,” and they do not affect current business operations in any way.

Teamsters said in a news release that members voted “by an overwhelming 97%” to allow a possible strike.

“This vote shows that hundreds of thousands of Teamsters are united and determined to get the best contract in our history at UPS. If this multibillion-dollar corporation fails to deliver on the contract that our hardworking members deserve, UPS will be striking itself,” Teamsters General President Sean O’Brien said in a statement. “The strongest leverage our members have is their labor and they are prepared to withhold it to ensure UPS acts accordingly.”

If a strike happens, it would be the largest strike of any U.S. industry since the 1950s; the last time UPS employees went on strike was 26 years ago, The Washington Post reported.

A strike threatens significant economic disruptions this summer. The company said it moves about 6% of the country’s domestic production, delivering 17 million packages a day across the nation. Even the slightest disruption threatens a major backup for the U.S. supply chain.

Typically, during the summer retailers are receiving shipments for back-to-school and holiday shopping. Losing UPS would almost surely slow that process down.

UPS and Teamsters have been negotiating since May. As part of the negotiations with UPS, the union is asking for higher full and part-time pay, more full-time jobs, better benefits, better working conditions and no more excessive overtime, among other things.

At least one agreement has been reached — UPS has agreed to install air conditioning units inside its entire fleet of 95,000 delivery trucks and vans.

Drivers have long complained about the hot conditions they have to endure, especially in the summer, and some workers have been hospitalized with heat-related illnesses.

In a statement, UPS said, “We are pleased we are making steady progress with the Teamsters on a wide array of topics. UPS is focused on reaching a deal that is a win for our employees, the teamsters, UPS and our customers.”

UPS leaders have assured customers deliveries will continue no matter what, but if history is any indication delivering packages during a strike will be challenging.

In 1997, about 185,000 UPS workers went on a two-week strike, which resulted in an estimated $620 million loss for the company. At the time, it was the biggest U.S. strike in decades.

The Associated Press contributed to this report.