Regulator Ofcom is facing a legal challenge over its ruling that Sky would remain "fit and proper" to hold a UK broadcasting licence if it was acquired by Rupert Murdoch's 21st Century Fox.
Campaign group Avaaz said on Friday that it is filing papers in court seeking a judicial review of the ruling, adding that it aims "to have Ofcom's decision quashed and a new investigation opened".
Alex Wilks, Avaaz campaign director, said: "Repeated, large-scale scandals in the Murdoch empire indicate that something is very rotten at the core of their businesses.
"Ofcom didn't dig deep enough before declaring the Murdochs fit to own even more of our media."
Ofcom told Culture Secretary Karen Bradley in June that the takeover attempt raised "public interest concerns" and she could refer the deal to the Competition and Markets Authority (CMA) for a fuller investigation on the grounds of media plurality.
But the watchdog said the £11.7bn merger would not stop Sky from holding a broadcasting licence.
Avaaz claims the watchdog's decision not to rule against the deal when it came to the UK broadcasting licence was based on "exaggerated fears of the consequences of doing so".
The activist group alleged Ofcom had set too high a bar for finding 21st Century Fox "unfit and improper" to hold a licence.
It also claimed the regulator had made errors when assessing 21st Century Fox's compliance with the UK broadcasting code, drew "wrong conclusions" from allegations of sexual and racial harassment at Fox News, and ignored the role James Murdoch would play as chief executive of 21st Century Fox.
An Ofcom spokesperson said: "We will defend our 'fit and proper' assessment, which was independent, expert and based on the evidence."
Earlier this month Ms Bradley confirmed the deal - which would see Mr Murdoch acquire the 61% of Sky that 21st Century Fox does not already own - will be referred to the competition watchdog for an in-depth probe.
