The Federal Trade Commission announced Monday that it would seek to block the merger between the two leading daily fantasy sites, FanDuel and DraftKings.

ALSO SEE For more coverage, see Outside The Lines' August 2016 feature -- Welcome to the Big Time -- on the rise and downfall of the daily fantasy sports industry, specifically DraftKings and FanDuel.

The regulatory body said it would file a suit, together with California and Washington, D.C., in an effort to temporarily stop the two, who own 90 percent of the daily fantasy business, from combining into what its officials believe would be an illegal monopoly.

"The proposed merger would deprive customers of the substantial benefits of direct competition between DraftKings and FanDuel," said Tad Lipsky, acting director of the FTC's Bureau of Competition.

The companies agreed to merge in November after the expensive effort to reign supreme (an estimated $750 million in marketing was spent by the two companies in fall 2015) and political battles with states as to whether daily fantasy was to be considered gambling.

"We are disappointed by this decision and continue to believe that a merger is in the best interest of our players, our companies, our employees and the fantasy sports industry," said DraftKings CEO Jason Robins and FanDuel CEO Nigel Eccles in a joint statement. "We are considering all our options at this time."

The trial, the FTC notes, would commence on Nov. 21.