Major League Soccer has informed the MLS Players Association that it will invoke a force majeure clause that obligates the parties to negotiate modifications to the existing collective bargaining agreement in good faith for 30 days, a source with knowledge of the situation told ESPN on Tuesday. If the sides are unable to reach an agreement, the CBA negotiated in June could be terminated.

The source added that MLS has not yet made a proposal to the players for a 2021 CBA, but that would happen "promptly."

"Unfortunately, based on the assessment of public health officials, it is clear that the impact of COVID-19 and the restrictions on attendance at sporting events will continue into the 2021 MLS season," deputy commissioner Mark Abbott told ESPN. "We recognize the impact that the pandemic has had on our players and appreciate their efforts to restart and complete the 2020 season, but, like the other leagues in the United States and Canada, MLS needs to address the ongoing challenges caused by the pandemic and will engage in good-faith discussions with our players about ways to manage the significant economic issues we are facing."

The league's owners and players had agreed to a framework on a new CBA in February following the expiration of the previous deal, but it wasn't ratified by either side. When the COVID-19 pandemic hit, the league was shut down on March 12, resulting a severe decrease in revenues. The lack of ratification provided MLS with an opening to renegotiate some of its terms.

Those terms included a 5% reduction in players' 2020 wages as well as a $5 million cap on performance and individual bonuses.

The MLS Players Association (MLSPA) condemned the move.

"After a 2020 season of extreme sacrifice, immeasurable risk to personal health, and a remarkable league-wide effort to successfully return to play, this tone-deaf action by the league discredits the previous sacrifices made by players and the enormous challenges they overcame in 2020," the MLSPA said in a statement.

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MLS commissioner Don Garber said that the league lost nearly $1 billion in revenue in 2020, and the source stressed to ESPN that the league couldn't afford a second successive season suffering such losses.

Its television rights deals with ESPN, Fox Sports and Univision are worth $90 million a season, making it heavily dependent on matchday revenue. And with National Institute of Allergy and Infectious Diseases director Dr. Anthony Fauci suggesting fans in large numbers may not be able to attend sporting events until the second half of 2021, MLS faces the prospect of another year of largely empty stadiums.

"We're gonna be vaccinating the highest-priority people [from] the end of December through January, February, March," Fauci said in a November interview with Yahoo Sports. "By the time you get to the general public, the people who'll be going to the basketball games, who don't have any underlying conditions, that's gonna be starting the end of April, May, June. So it probably will be well into the end of the summer before you can really feel comfortable [with full sports stadiums] -- if a lot of people get vaccinated. I don't think we're going to be that normal in July. I think it probably would be by the end of the summer."

Only a handful of MLS' 26 clubs allowed fans after the pandemic hit. The league also incurred several added costs in 2020, with the expense of hosting the MLS is Back Tournament in a bubble in Orlando, as well as transporting teams to away games on charter flights to complete its regular season after the July competition in Central Florida.