When Rupert Murdoch took over the Wall Street Journal, the joke was that pictures of topless women would soon appear in the stipple portrait style that has long graced the pages of the business newspaper.

At the US Supreme Court today, the two sides of Mr Murdoch’s News Corporation – serious news and entertainment – will be front and centre once again, this time as the company argues a case experts say represents the most important test of free speech to be heard by the court in three decades.

In FCC v Fox Television Stations, the high court will rule on whether a policy governing the airing of “fleeting expletives” on broadcast television, enacted by the Federal Communications Commission, is “arbitrary and capricious”.

The Supreme Court took the case after a lower court ruled in June 2007 that the FCC, the federal media regulator, had failed to provide a “reasoned explanation” for a change in its policy when it fined News Corp’s Fox Network after it aired obscenities by Cher and Nicole Ritchie in a 2002 broadcast of the Billboard Music Awards.

But in taking the case, some experts predict that the Supreme Court might set a precedent beyond the ­narrow issue of indecent ­language on live television. They say the case could spur a broader review of how broadcast television is regulated by indecency provisions in the 21st century, when unregulated cable channels are a click away on the remote control, as is the unregulated internet.

At the time of the lower court ruling in favour of Fox, Kevin Martin, FCC chairman, said that if the regulator was unable to restrict the use of obscenities in primetime, “Hollywood will be able to say anything they want, whenever they want”. Mr Martin is supported by Christian conservatives, who defend indecency rules, but some former FCC commissioners, including a Reagan-appointed chairman, have filed briefs to the court chastising him.

“In pursuit of a policy of protecting children against exposure to extremely offensive language, the FCC has embarked on an enforcement programme that has all the earmarks of a Victorian crusade,” the former officials said.

Peter Chernin, News Corp’s president, admitted that the indecency case was not what most people would consider an important milestone in the fight for free speech. “If anyone had told me that my company would be before the US Supreme Court defending inane comments by Cher and Nicole Ritchie, I would have said you’re crazy,” he said. “But I would contend that the nature of this speech, and who said it, makes absolutely no difference.

“If the government gets its foot in the censorship door with respect to unpopular entertainment content, it is the beginning of the steep slide toward censoring unpopular political content.”