The editor of the Mail Online is to give evidence to the Leveson Inquiry, 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.
Mr Coulson, who appears on Thursday, 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 on Tuesday, 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 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.