Bettors have wagered over $1.2 billion in New Jersey on athletic contests since the legal sports betting launched in June, but the rapid growth of bookmaking appears to have hit a plateau in December, according to a monthly report released Monday by the New Jersey Division of Gaming Enforcement.

Bookmakers reported $20.8 million in gross revenue in December, down 2 percent from $21.2 million in November. Bettors wagered $319.2 million in December, down 3.4 percent from $330.7 million bet in November. About 75 percent of the sports bets in December were made online, rather than in person at casinos or racetracks.

The flattening growth in sports wagering coincides with the launch in December of sports betting at SugarHouse Casino in Philadelphia, which may be siphoning off some of the action that previously went to New Jersey. Sports betting launched at a second Philadelphia-area venue, Parx Casino in Bensalem, last week, and is set to start Tuesday at the South Philly Turf Club on Packer Avenue.

Two other Philadelphia casinos — Harrah’s Philadelphia Casino & Racetrack in Chester and Valley Forge Casino Resort in King of Prussia — also plan to open sports betting in the coming months.

Despite the cross-border competition, the $94 million in revenue taken by New Jersey sportsbooks since June represents a significant share of the industry’s growth in New Jersey in 2018, when overall gaming revenue was $2.903 billion, up $244 million, or 9.2 percent, from 2017. Casino win was $2.511 billion, up 4 percent from 2017, and internet gaming revenue increased 21.6 percent to $298.7 million in 2018.