Major League Soccer's referees and other on-field officials have been locked out, a day before the season openers.

The Professional Referee Organization, which manages game officials for the U.S. Soccer Federation and MLS, notified the Professional Soccer Referee Association of the lockout Friday. PRO says replacement officials will be used.

PRO says the union, which was formed last year, rejected a no strike-no lockout agreement that management had proposed.

The sides have bargained for months without reaching a deal on a labor contract.

More From ESPN Western Colorado