The competition watchdog has provisionally found that 21st Century Fox's proposed £11.7 billion takeover of Sky is not in the public interest.
:: What is the competition watchdog worried about?
The Competition and Markets Authority (CMA) has dealt a bloody nose to Rupert Murdoch in his attempts to push through a tie-up between 21st Century Fox and Sky.
Its concerns centre on "media plurality" - or the fear that one organisation would wield too much power over the political process if it holds a large slice of the Britain's media industry.
Rupert Murdoch's UK media empire is already vast. He owns News UK, home to newspapers The Sun, The Times, The Sunday Times, The Times Literary Supplement, Wireless Group and Unruly.
Parent company News Corporation also holds news wire service Dow Jones and the Wall Street Journal.
So a deal for Fox to buy the 61% of Sky it does not already own, including Sky News, is seen as a step too far for the CMA.
:: Does it mean the deal is off?
The CMA's provisional findings are a setback for Fox's takeover of Sky, but the deal is not dead in the water.
The watchdog has tabled three options which would help put its mind at rest.
It could block the deal, Sky News could be spun off, or "behavioural" changes could be made to protect Sky News from the direct influence of the Murdoch Family Trust.
:: Where does Walt Disney fit into all this?
Walt Disney announced a 52.4 billion US dollar (£39 billion) deal to buy 21st Century Fox's entertainment assets last month.
The move handed the entertainment giant a hefty slice of Mr Murdoch's empire, including Fox's 39% stake in Sky.
The takeover was widely seen as a positive step for the Fox-Sky deal because Disney would ultimately become the sole owner of both Sky and Sky News.
The watchdog noted the deal in Tuesday's finding, saying the Disney takeover would "significantly weaken" the link between Sky News and the Murdoch Family Trust.
But on a more cautious note, it said there was not enough certainty that the Disney-Fox deal would go through - and if so, when it might happen.
Problems would arise, for example, if the Disney-Fox deal was not complete before the Fox-Sky takeover was pushed through.
The watchdog has flagged the possibility of a so-called sunset clause, whereby moves to satisfy competition concerns would fall away if the Disney-Fox deal went through as planned.
:: What happens next?
The CMA's provisional findings may be a significant moment, but there is still a long road ahead.
A consultation on the watchdog's remedies closes on February 6, while an over-arching consultation on the provisional findings ends on February 13.
The CMA then has until May 1 before its final report is sent to Culture Secretary Matt Hancock. He will then have 30 working days to consider them before making a final decision on the deal.
:: What happened last time?
Mr Murdoch's move for Sky comes after his last attempt at seizing control of the business through News Corporation in 2011.
The tilt faced opposition from media industry rivals and politicians before it was scuppered by acute pressure on the company brought about by phone-hacking claims involving News International.