The US Federal Trade Commission (FTC) said on Monday it has authorized legal action “to block the merger” of Edinburgh-headquartered FanDuel and Boston-based DraftKings, the two largest daily fantasy sports sites.

The FTC alleges that the combined firm would control more than 90% of the US market for paid daily fantasy sports contests.

The FTC said it will, jointly with the offices of the attorneys general in the State of California and the District of Columbia, file a complaint in federal district court seeking a preliminary injunction to stop the deal and to maintain the status quo pending an administrative trial.

“This 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 FTC is committed to the preservation of competitive markets, which offer consumers the best opportunity to obtain innovative products and services at the most favorable prices and terms consistent with the provision of competitive returns to efficient producers.”

FanDuel CEO Nigel Eccles and DraftKings CEO Jason Robins issued a joint statement saying: “Today, the Federal Trade Commission (FTC) announced it will attempt to block the proposed merger between DraftKings and FanDuel.

“We are disappointed by this decision and continue to believe that a merger is in the best interests of our players, our companies, our employees and the fantasy sports industry.

“We are considering all our options at this time.”

The FTC said it also issued “an administrative complaint alleging that the proposed merger violates Section 7 of the Clayton Act and Section 5 of the FTC Act.”

The FTC added: “The Commission votes to issue the administrative complaint and to authorize staff to seek a temporary restraining order and preliminary injunction in federal court were both 2-0.

“The federal district court complaint was filed on June 19, 2017 in the U.S. District Court for the District of Columbia. The administrative trial is scheduled to begin on November 21, 2017.”