>> Top British police chief steps down over hacking scandal

With a second British police chief quitting on July 18 over the scandal, Cameron cut short a trade trip to Africa and was due to return late on July 19 to attend an emergency debate the following day in parliament, which is delaying its summer recess. Before returning to Britain the prime minister, in Lagos, Nigeria, will give a press conference at 1200 GMT (8 a.m. ET).

The Murdochs' appearance before parliament's media select committee was expected to attract a television audience of millions keen to follow the latest twist in a saga that has shaken Britons' faith in their police, press and political leaders.

"It seems as if there will be standing-room only, that's not surprising as it's the first time Rupert Murdoch has been before a select committee in his 40 years of building up a media empire," said Paul Farrelly, an opposition Labour committee member.

Murdoch's News International British arm had long maintained that the practice of intercepting mobile phone voicemails to get stories was the work of a sole "rogue reporter" on the News of the World newspaper.

That defense crumbled in the face of a steady drip-feed of claims by celebrities that they were targeted.

The floodgates opened two weeks ago when a lawyer for the family of a murdered teenage schoolgirl claimed the paper had hacked her phone when she was missing, deleting messages and raising false hopes she could be still alive.

The ensuing outrage prompted News Corp to close the 168-year-old News of the World newspaper, drop a US$12 billion plan to take full control of pay TV operator BSkyB, and saw the arrest of former News International Chief Executive Rebekah Brooks, a Murdoch protégé.

Cameron has faced questions over his judgment in appointing former News of the World editor Andy Coulson as his communications chief, while London police chief Paul Stephenson and anti-terrorism head John Yates stepped down within 24 hours of each other over their links to a former deputy editor of the newspaper.