The editor of the Mail Online will give evidence to the Leveson Inquiry today, ahead of potentially explosive appearances by former News International executives Rebekah Brooks and Andy Coulson later this week.
The inquiry into press standards will hear evidence from Martin Clarke, as well as from officers about police corruption investigations at Scotland Yard and Devon and Cornwall Police, today.
Mr Coulson, who appears tomorrow, and Mrs Brooks, due to give evidence on Friday, are expected to make embarrassing revelations about British politicians' attempts to woo Rupert Murdoch's newspapers.
Mrs Brooks is likely to disclose further details about her close relationship with the Prime Minister, while former News of the World editor
Mr Coulson will speak about how he came to be appointed the Tories' top spin doctor.
Their evidence threatens to overshadow David Cameron's efforts to relaunch the coalition after bruising local election results.
On a joint visit yesterday, he and Deputy Prime Minister Nick Clegg said they were both committed to sorting out the British economy.
Eight Cabinet Ministers - including Mr Cameron, Deputy Prime Minister Nick Clegg and Culture Secretary Jeremy Hunt - were last week given the right to see Leveson Inquiry documents and witness statements in advance after Lord Justice Leveson agreed to make them "core participants".
Mr Coulson's appearance before the inquiry will revive awkward questions about Mr Cameron's decision to make him his communications director.
The Prime Minister said last July that "with 20:20 hindsight" he would not have hired Mr Coulson in May 2007, four months after he resigned from the Sunday tabloid over the jailing of royal reporter Clive Goodman for phone hacking.
Mr Coulson, 44, became Downing Street's communications chief in May 2010 but quit eight months later, saying controversy over the hacking scandal was making his job impossible.
His Leveson Inquiry evidence will be the first time he has spoken publicly since being arrested by Scotland Yard on suspicion of phone hacking and corruption last July. He was bailed and has not been charged.
Mrs Brooks, 43, edited the News of the World and The Sun before becoming chief executive of Mr Murdoch's UK newspapers division News International in September 2009.
She and racehorse trainer husband Charlie are key members of the influential Chipping Norton set, which also includes Mr Cameron and his wife Samantha, Top Gear presenter Jeremy Clarkson, and Mr Murdoch's daughter Elisabeth and her PR guru husband Matthew Freud.
The inquiry has already heard that Mrs Brooks regularly met Mr Cameron and other top politicians along with Rupert and James Murdoch.
She hosted a Christmas dinner on December 23 2010, just two days after Business Secretary Vince Cable was stripped of his responsibility for media takeovers for saying he had "declared war" on the Murdochs' News Corporation empire.
James Murdoch told the inquiry he had a "tiny side conversation" with Mr Cameron before the meal about Mr Cable's comments and News Corp's bid to take over broadcaster BSkyB.
Mrs Brooks's wedding on June 13 2009 was attended by Mr Cameron and former prime minister Gordon Brown, and Mr Cameron was forced to admit in March that he rode a retired police horse loaned to Mrs Brooks by Scotland Yard from 2008 to 2010.
There is speculation that the Leveson Inquiry could release emails and text messages sent between Mr Cameron and the former News International chief executive.
Mrs Brooks has kept all the texts she received from the Prime Minister, of which there could have been more than 12 a day, according to Daily Telegraph columnist Peter Oborne.
Emails sent by News Corp lobbyist Frederic Michel released by the inquiry led to calls for Mr Hunt's resignation over claims he secretly backed the Murdochs' BSkyB takeover bid.
Mrs Brooks has twice been arrested by Scotland Yard detectives investigating allegations of phone hacking, corrupt payments to public officials, and an attempt to pervert the course of justice. She was bailed and has not been charged.
Leveson Inquiry lawyers will not question Mrs Brooks or Mr Coulson about anything that could prejudice the continuing police investigation into phone hacking or any potential future trials.
Further testimony tomorrow will come from Lord Rothermere, chairman of Daily Mail and General Trust, which owns the Daily Mail and Mail on Sunday.
Yesterday the inquiry denied asking George Michael to give evidence after the star claimed he was asked but declined, dismissing it as a sham.
Mr Cameron set up the Leveson Inquiry last July in response to revelations that the now-defunct News of the World hacked murdered schoolgirl Milly Dowler's phone after she disappeared in 2002.
The first part of the inquiry, sitting at the Royal Courts of Justice in London, is looking at the culture, practices and ethics of the Press in general and is due to produce a report by October.
Lord Justice Leveson indicated last week that the second part, examining the extent of unlawful activities by journalists, may not go ahead.