Heather Mills is to give evidence at the Leveson Inquiry into press standards, it has emerged.
The ex-wife of Sir Paul McCartney is expected to answer questions relating to evidence from former News of the World editor Piers Morgan.
Daily Mail editor Paul Dacre, who appeared on Monday, will also return to the inquiry tomorrow.
When giving evidence in December, Morgan told the inquiry he would not disclose a source who played him a tape of a message that Sir Paul left Mills.
He said: "I am not going to discuss where I heard it or who played it to me.
"I don't think it's right. In fact the inquiry has already stated to me you don't expect me to identify sources."
Lord Justice Leveson told Morgan the only person who would be able to lawfully listen to the message was Mills or somebody authorised on her behalf.
He told Morgan: "I am perfectly happy to call Lady McCartney to give evidence as to whether she authorised you to listen to her voicemails.
"She may say she did in which case you're not compromising anybody, but if she didn't then we can proceed on the premise that it's somebody else, can't we."
Ms Mills is expected to appear at the inquiry, sitting at the Royal Courts of Justice in London, tomorrow morning.
Also appearing tomorrow is Paul Dacre, editor-in-chief of Associated Newspapers, which publishes the Daily Mail and The Mail on Sunday, who has been recalled to the inquiry.
He will answer more questions about accusations made by actor Hugh Grant.
On Monday the editor defended accusing Mr Grant of "mendacious smears" after the actor claimed a Mail on Sunday story about his relationship with socialite Jemima Khan was likely to have come from illicit eavesdropping.
After Mr Grant's claims during his evidence at the start of the inquiry, Associated Newspapers issued a statement branding his claims "mendacious smears driven by his hatred of the media".
Mr Dacre told the inquiry he was off work the day the claims were made but heard reports of them on BBC radio.
"It was a terrible smear on a company I love," he said. "We had to do something about it."
He said the comments were an attempt by Grant, the Hacked Off campaign and the Media Standards Trust, to "hijack" the inquiry and if Associated had not instantly strongly rebutted the allegations, it would have been "too late".
"My company would have been smeared, my newspapers would have been smeared and I was not prepared to allow that," he said.
"I will withdraw that statement if Mr Grant withdraws his statements that the Daily Mail and Mail on Sunday were involved in phone hacking.
"I have never placed a story in the Daily Mail as a result of phone hacking, that I know came from phone hacking. I know of no cases of phone hacking.
"Having conducted a major internal inquiry, I am as convinced as I can be that there is no phone hacking on the Daily Mail.
"I don't make that statement lightly. And no editor, the editor of the Guardian or the Independent, could say otherwise."
Mr Dacre said a newspaper was entitled to ask a celebrity like Grant if he had had a child, especially when he had spoken previously of his desire to be a father.
"Mr Grant has spent his life invading his own privacy," he said.
"It seems a little bit ripe that when he does have a child, he and his press representatives won't confirm or deny that."
In a statement after his evidence, the Hacked Off campaign and Media Standards Trust rejected his claims.