Former Daily Mirror and News of the World (NoW) editor Piers Morgan is set to appear at the Leveson Inquiry into media ethics later.
Mr Morgan now works for CNN in the United States and will appear at the inquiry in London via video link.
He has previously denied ever hacking a phone, telling someone else to, or publishing a story obtained by hacking.
Former NoW TV editor Sharon Marshall, lawyer Julian Pike, media agency heads and a union chief are also to appear.
Heather Mills' voicemail
Mr Morgan was the Mirror's editor between 1995 and 2004 and also edited the NoW between January 1994 and November 1995.
In August, it emerged that Heather Mills - former wife of Sir Paul McCartney - had alleged that a senior Mirror Group journalist admitted to her in 2001 that he had hacked into her voicemail.
The senior journalist referred to by Ms Mills is known not to be Mr Morgan. But the message in question appeared to be the same as one that he admitted to having listened to in a Daily Mail article in 2006.
In a statement issued in August, he said Ms Mills' claims were "unsubstantiated", adding: "To reiterate, I have never hacked a phone, told anyone to hack a phone, nor to my knowledge published any story obtained from the hacking of a phone."
'Routine' phone hacking
On Monday, Stuart Hoare - brother of former NoW journalist Sean Hoare - told the inquiry his brother had witnessed daily phone hacking at the News of the World and routine phone hacking at the Sun.
Before his death earlier this year, the journalist had told the BBC's Panorama the then NoW editor Andy Coulson had asked him to hack phones - something Mr Coulson has denied.
In his evidence to the inquiry, James Hanning, deputy editor of the Independent on Sunday, said while Mr Hoare was aggrieved at having been sacked, he did not think it was Mr Hoare's "prime spur" for speaking out.
Meanwhile, former NoW sports reporter Matt Driscoll, who won an employment tribunal claim for disability discrimination after he was sacked by the paper, spoke of a culture of bullying.
Mr Driscoll also explained how "blagging" could be used to support a story, using an example of medical records being accessed.
Prime Minister David Cameron set up Lord Justice Leveson's inquiry after the NoW admitted intercepting voicemail messages of prominent people to find stories.