European Commission investigators raided Twenty-First Century Fox's London office on Tuesday, The Daily Telegraph reported.

Sources at the location told the newspaper that authorities were trying to be discreet and had been told to keep details of their mission confidential. The Telegraph reported that investigators will be on site through Wednesday and maybe Thursday.

Shares of Fox fell more than 1 percent in extended trading.

The raid comes amid Fox's drawn out takeover of Sky, which has fallen under intense scrutiny by regulators at the U.K.'s Competition and Markets Authority.

Earlier this year, the agency provisionally found Fox's bid would not be in the public interest. It said there are concerns that the deal may lead to the Murdoch Family Trust wielding too much influence over public opinion and the political agenda.

Fox Networks Group said in a statement to CNBC that it is "cooperating fully with the EC inspection."

The European Commission confirmed to CNBC that it carried out unannounced inspections of companies that distribute sports media rights in several countries on Tuesday.

The Commission said the affected companies may have violated antitrust rules that bar cartels and restrictive business practices. It said the inspections were carried out by its investigators and their local counterparts from relevant authorities in those countries.

While such inspections are part of the Commission's process of looking into allegations of anticompetitive practices, it emphasized that the relevant companies have a right to be heard in proceedings.

The Commission did not list which companies had their offices inspected on Tuesday.

Read the full report in The Daily Telegraph.