Major League Soccer’s playoff format will change again in 2015, when 12 teams will vie for the MLS Cup trophy rather than the current 10. An announcement could come as early as next week, according to a source with knowledge of the decision.

This year has been the fourth in which 10 clubs have qualified for the postseason and the third featuring a 19-team regular season circuit. But the league is growing.

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There will be 20 clubs in 2015 (thanks to the departure of Chivas USA and the addition of Orlando City and New York City FC), then 22 in 2017 when Atlanta and Los Angeles FC come aboard. By the time MLS reaches 24 clubs in 2019-2020, the playoff qualification rate will have fallen back to 50 percent -- assuming there are no subsequent changes.

The new MLS Cup format calls for an increase in knockout-round matches from two to four.

Under the current system, the fourth- and fifth-place finishers in each conference meet in a one-game playoff that sends the winner to face the top seed. Starting next season, the third-place team in each conference will host the sixth seed in one single-elimination game while No. 4 hosts No. 5 in the other. The four conference semifinalists then will be reseeded to ensure the first-place finisher faces off against the lowest-ranked knockout round survivor. The bracket will look like the one used in the NFL since 1990.

The Eastern and Western conference semifinals and finals will remain as two-game series featuring the away-goals tiebreaker. The MLS Cup final will remain a single match.

Next season’s 60 percent MLS playoff qualification rate (and the 50 percent rate expected in 2020) compares to 53.3 percent in the NBA and NHL, 37.5 percent in the NFL and 33.3 percent in Major League Baseball.