It was the day the watchdog finally revealed its fangs to the neighbourhood tough guy. For months there has been a low-level growl of disapproval from Ofcom, sniffing around in the shape of its preliminary inquiry into whether News Corp is "fit and proper" to have access to the British television sector as the major shareholder in the satellite broadcaster BSkyB.

But yesterday the watchdog broke its chain and went on the attack. The News Corp subsidiary News Group Newspapers (NGN) was ordered to hand over documents that had not been in the public domain. The Ofcom inquiry was ratcheted up from a "monitoring phase" to a far more serious "evidence-gathering phase".

Ofcom appears to have scented something; the paperwork that NGN must disclose is potentially explosive as it relates to the civil litigation cases over phone-hacking that News Corp has been trying to settle out of court. It is understood that the company will comply with the request.

This week the watchdog has been sorely provoked. Internal News Corp emails disclosed to the Leveson Inquiry on Tuesday showed the regulator was being talked of with disdain in exchanges between the company and Jeremy Hunt, who apparently accused Ofcom of "clear bias" over News Corp's bid to take control of BSkyB.

The demand for unseen documents is seen as a pivotal moment in broadcast regulation. "It's a fantastic step," said Steven Barnett, Professor of Communications at the University of Westminster. "A year ago Ofcom would not have had the corporate courage to have done something like this because they would have been slaughtered. This is the regulator doing what it should have been doing all along and flexing its muscles."

Rupert Murdoch may not have completely abandoned his ambitions to own the highly profitable BSkyB. But the latest development increases the possibility that his company will lose even the lucrative 39.1 per cent share in the broadcaster that it still has (an unthinkable situation last year when the bid looked set to be approved by Mr Hunt). Further criticisms of News Corp's corporate behaviour are expected in an imminent report by the House of Commons Culture committee.

The American authorities will be watching all these developments closely, said Bob Calver, senior broadcast lecturer at Birmingham City University. “If the Murdoch empire is not seen as fit and proper to own a broadcast licence in the UK, the American regulators and investigators will wonder what that says about their fitness to operate in the US.”