The growth in TV advertising revenue last quarter was driven largely by daily fantasy sports sites, which are under legal scrutiny in New York, according to media analyst Todd Juenger.
Almost 60% of the sales growth in the third quarter -- or $134 million of total growth of $227 million -- came from daily fantasy sports companies, Mr. Juenger, at Sanford C. Bernstein & Co., wrote in a research note Wednesday.
On Tuesday, New York Attorney General Eric Schneiderman sued FanDuel and DraftKings, the startups that dominate the daily fantasy sports industry, arguing they're a form of illegal gambling and should be shut down in the state. On Nov. 25, a judge in Manhattan will decide whether their sites can continue operating.
The influx in advertising helped boost revenue at media companies with sports programming, like CBS, Walt Disney and 21st Century Fox, Mr. Juenger said. Fantasy sports companies claimed so much TV ad inventory last quarter that they also drove up prices across the industry, even helping media companies without sports programming, like AMC Networks, he said.
The amount of TV ad spending from fantasy sports companies "is unusual for both its size and its questionable durability," Mr. Juenger said.
"Daily fantasy sports sites may not even exist next year," Mr. Juenger said. "Even if they do, the probability that they grow again at these rates is very low."
The sites have been flooding the airwaves with their get- rich-quick commercials, mostly on sports networks. Earlier this fall, DraftKings and FanDuel were among the Top 10 biggest spenders on TV advertising, but have since dropped off that list, according to ad tracker iSpot.tv.
FanDuel's advertising fluctuates based on the sports calendar, said Justine Sacco, a spokeswoman for the company.
"It spikes at the beginning of football season when we acquire the majority of our new users and declines as the season progresses, so you will see a strategic decrease in spend moving forward," Ms. Sacco said in an e-mail.
A representative for DraftKings didn't immediately respond to a request for comment.
So far this year, DraftKings has spent more than $154 million on almost 46,000 TV commercial spots, while FanDuel has spent almost $112 million on more than 27,000 commercial airings, according to iSpot.tv.
Both companies are valued at more than $1 billion each and have raised a combined $575 million in funding through investors that include several big media companies, including Fox, Time Warner and Comcast, according to Bloomberg Intelligence.
FanDuel CEO Nigel Eccles told Bloomberg earlier this month that his company will focus less on television in the future and more on cheaper, more targeted digital ads. DraftKings has asked some TV networks to delay its ad commitments as it faces legal challenges, the Wall Street Journal reported Monday, citing people familiar with the matter.
TV networks owners "should be concerned" if regulators crack down on the daily fantasy sports business, noting its impact not only on ad spending but also on ratings, Michael Nathanson, an analyst at MoffettNathanson, wrote in a note Wednesday.
"Killing the golden goose of daily fantasy sports would definitely be against the interest of the NFL's network partners," Mr. Nathanson wrote.