But the regulators also offered ways for Fox to remove their objections, including a spin-off of Sky News or "behavioural remedies" that would reduce the Murdoch family's ability to direct the channel's news coverage.
Fox said it was "disappointed" by the ruling on media plurality, while Sky took note of the suggested remedies.
The authority recognised that the completion of Disney's bid for Fox would weaken concerns about media plurality because Disney would own Sky. But the authority said there was no way to guarantee when or if Disney's takeover of Fox will be completed so the British government's review of the Sky merger must go forward.
"We cannot be sufficiently confident at this stage whether, when, or how the Disney/Fox transaction will complete," the regulator said.
Former British Culture Secretary Karen Bradley asked the authority to evaluate the takeover in September, directing it to look at Fox's commitment to broadcasting standards and the deal's impact on media plurality in the U.K.
In its decision, the regulator dismissed concerns about broadcasting standards, saying that Fox and Sky had a good record in this area. As part of its investigation, the authority considered allegations of sexual harassment at Fox News in the US.
"While these are serious, the CMA has provisionally found that these are not directly related to the attainment of broadcasting standards and do not call into question Fox's or the (Murdoch family trust's) commitment to broadcasting standards in the UK," the authority said.
Campaign group Avaaz did not bother to hide its disappointment that the authority did not hold the Murdoch empire to account for alleged sexual harassment in the workplace and other issues. They promised to fight on.
"The watchdog has let the Murdochs squirrel out of any responsibility for the hacking, harassment and hush money at his companies," Alex Wilks, campaign director, Avaaz. "In the coming weeks they'll hear from citizens, victims and whistleblowers to show them they got this wrong."
Analysts said the ruling may actually be good news for the takeover because the spinoff of Sky News is workable — while it would have a less clear path for the companies to mitigate concerns about broadcasting standards. Shares of Sky plc rose 2.3 percent in London.
"It has a path to conclusion," said media analyst Alice Enders of the decision. "That's why the markets are so jolly."
Fox is seeking to consolidate its control over Sky as media companies try to combine content creation and distribution channels amid pressure from competitors such as Netflix, Google and Amazon.
Sky's European pay TV operation has 22.5 million customers, attracted by offerings such as English Premier League soccer and Game of Thrones.
A previous bid for the whole of Sky foundered amid the 2011 phone-hacking scandal, in which journalists working for Murdoch newspapers were accused of gaining illegal access to the voicemail messages of celebrities, members of the royal family and crime victims. Murdoch's News Corp. withdrew its bid for Sky in 2012.
Labour Party Deputy Leader Tom Watson, a long-time opponent of the Murdochs, tweeted that the regulator was "right to say that the Fox takeover of Sky would give the Murdoch family too much power."
"This is the right decision for the UK," he said.