The Leveson inquiry into the ethics of the British press opened on Monday with the judge vowing to find out "who guards the guardians" in the wake of the News of the World phone-hacking scandal.

Some four months after a flood of hacking allegations shocked the nation and prompted the closure of the Rupert Murdoch-owned News of the World, the public inquiry into the press led by judge Brian Leveson formally got under way.

Prime Minister David Cameron ordered the investigation after the current system of self-regulation failed to identify rogue practices at the Sunday tabloid, which was Britain's biggest-selling weekly newspaper.

The controversy has rocked Murdoch's News Corp. media empire and prompted criticism of the relationship between the press, politicians and police, whose initial investigation failed to uncover the extent of the wrongdoing.

Opening the inquiry, Leveson said a central question of the probe was who guarantees that the media, the guardians of democracy, are themselves held to account?

"The press provides an essential check on all aspects of public life. ... Part of this inquiry therefore may be one simple question -- who guards the guardians?" he said.

The first part of the inquiry will focus on the culture, practices and ethics of the press in general, and the opening session will include a statement from the lawyer of murdered schoolgirl Milly Dowler's family.

Revelations that Dowler's phone voicemail was illegally accessed by a private detective working for the News of the World made the long-simmering scandal explode into a crisis, prompting Murdoch in July to shut the paper.

Dowler's family are among 50 "core participants", which include newspaper groups as well as people who have complained about press intrusion.

The inquiry's main counsel, Robert Jay, will make the opening statement at the Royal Courts of Justice in London. The first witnesses will not be called until next week.

Preliminary seminars have already been held in recent weeks.

The second part of the probe will examine the extent of unlawful activities by journalists, including at the News of the World and its parent company News International.

However, this part of the probe cannot start until police investigations are complete.

Scotland Yard is running two investigations, one into phone hacking and the other into allegations of illegal payments made by journalists to police officers.

Police have made a string of arrests, although no one has yet been charged.

Those detained include former News of the World editors Rebekah Brooks -- later News International's chief executive -- and Andy Coulson, who served as the prime minister's media chief until January this year.

But doubts have been raised over Leveson's failure to appoint any representatives from the popular press to the panel of six experts advising him.

The panel includes a former political editor of the broadsheet Daily Telegraph and the former chairman of the Financial Times.

"With such a very large part of the industry unrepresented, I think there's a risk it will not look at the whole picture," Trevor Kavanagh, associate editor of Britain's top-selling paper, the Murdoch-owned Sun tabloid, told the BBC.

Coulson was editor of the News of the World when a royal reporter and a private detective were jailed for hacking in 2007. He resigned but denied any knowledge of hacking.

Over the past four years, however, it has become clear that thousands of people were targeted by the tabloid.

The story has refused to go away, with new allegations last week that News International spied on lawyers of hacking victims and Rupert Murdoch's son and heir James giving evidence on the scandal to lawmakers for a second time.