Three national newspaper editors from Trinity Mirror, one of Britain's largest media groups, will give evidence to the Leveson Inquiry today.
The press standards inquiry will hear from Richard Wallace of the Daily Mirror, the Sunday Mirror's Tina Weaver and Lloyd Embley of the People.
Further testimony will come from Sly Bailey, chief executive of Trinity Mirror, which also publishes two Scottish national titles and more than 160 regional newspapers.
The inquiry will receive evidence tomorrow from Private Eye editor Ian Hislop as well as Guardian editor Alan Rusbridger, Times editor James Harding and Sunday Times editor John Witherow.
On Wednesday the witnesses will be the editors of eight regional newspapers and celebrity magazines Hello!, OK! and Heat.
Prime Minister David Cameron set up the Leveson Inquiry last July in response to revelations that the News of the World commissioned a private detective to hack 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 September.
The second part, examining the extent of unlawful activities by journalists, will not begin until detectives have completed their investigation into alleged phone hacking and corrupt payments to police, and any prosecutions have been concluded.