The public confirmation of his plot is a direct challenge to Murdoch and Disney chairman Bob Iger as they prepare to take their deal to shareholders.
The three sides have fought multiple battles over the decades. An attempt by Murdoch to challenge US cable operators with a takeover of the satellite provider DirecTV in 2001 was met by Roberts with aggressive expansion and the takeover of AT&T Broadband. Three years later Comcast made a failed bid for Disney.
Fox's asset sale to Disney last year signalled a withdrawal by Murdoch. As the 87-year-old gradually hands power to his eldest son Lachlan, they are planning a much smaller "New Fox" focused on US news and sport.
The family will also remain in control of News Corp, the separately listed publisher of newspapers including the Sun and the Wall Street Journal. Prior to the agreement with Disney, Fox rejected rival interest from Comcast as too risky even though its bid was higher.
US regulators and the Trump Administration are opposing the planned takeover of Time Warner by the telecoms giant AT&T.
Unlike Comcast and AT&T, Disney does not operate in the broadband market and was seen as less likely to trigger competition concerns as owner of the Fox assets.
Roberts has refused to walk away, however. The decision by the Murdochs to retrench as the vast scale of internet operators such as Apple and Amazon cast a shadow is viewed as a rare opportunity to seize a greater share of the entertainment landscape.
The 58-year-old, who inherited leadership of Comcast from his father, its founder, first launched a bid to disrupt Fox's takeover of Sky to prevent Disney getting its hands on one of the jewels in the Murdoch crown.
Now, amid mounting speculation that a US court will cast aside Department of Justice objections to the AT&T takeover of Time Warner next month, he has also prepared an attack on a bigger deal between the Murdoch family and Disney.
Comcast's separate bid for Sky will go ahead. The Culture Secretary Matt Hancock this week indicated it will not trigger a media plurality -investigation, clearing the path for a bidding war with Fox and Disney.
Murdoch is awaiting the outcome of a long-running investigation of his sway over the British media before he can improve Fox's bid, which is currently 16 per cent lower than Comcast's.