London (CNN) -- Voice mails left by Paul McCartney for his then-girlfriend Heather Mills were illegally accessed and heard by a former employee of a major British newspaper group, Mills told an independent investigation into press ethics Thursday.
The former Beatle left about 25 messages for her one night in 2001, including one where he sang to her, as he tried to make up with her after a quarrel, she said.
"One of them said, 'Please forgive me,' and he sang a little ditty of one of his songs onto the voice mail," said Mills.
The former Trinity Mirror group employee later phoned her and said he had heard her voice mail.
She responded angrily, she testified before the Leveson Inquiry, saying there was no way he could have heard the message unless he had obtained it illegally.
Mills said the man laughed, and she said she told him: "I promise you, if you report this story, even if it's true, you have obtained the information illegally, and I will do something about it."
"And he never reported the story," she added.
The inquiry is not releasing the name of the person who heard the message because he is under police investigation, said Robert Jay, the chief counsel to the Inquiry.
But it was not a Daily Mirror journalist or anyone working under the supervision of its then-editor, Piers Morgan, Jay said. Morgan now hosts a CNN talk show, "Piers Morgan Tonight."
The voice mail in question was a critical point when Morgan testified in December before the Leveson Inquiry.
The probe was set up in response to widespread anger in Britain at the revelation that a murdered 13-year-old girl's phone was hacked by journalists in search of stories -- and that many other crime and terror victims, politicians and celebrities had also been targeted.
Rupert Murdoch's News of the World tabloid has been the focus of much of the anger, and Murdoch's son James ordered it shut down over the scandal.
But accusations have been leveled against other newspapers as well, including the Daily Mirror.
Testifying in December, Morgan said he did not believe phone hacking had taken place when he was editor of the tabloid, prompting Jay to follow up: "You don't believe so or you are sure?"
"I don't believe so," Morgan responded by video link.
Jay pressed Morgan particularly hard about his having written in 2006 that he had heard a message McCartney left for Mills, trying to make up after a quarrel and singing to her.
Morgan refused to say who played the message for him or where, but he admitted under sustained questioning that he believed it was a voice mail.
"Did you know that was unethical?" Jay demanded.
"Not unethical, no. It doesn't necessarily follow that it was unethical," Morgan said, insisting he would not "go down a trail that will lead to the identification of a source."
On Thursday, Mills said she had never authorized Morgan to access her voice mail or to listen to a recording of it.
"Never, ever," she said.
She said she had never made a recording of any of the messages.
"No, no. They were deleted pretty much straight away," she said.
Brian Leveson, the judge leading the inquiry, intervened to ask if she had ever authorized anyone to listen to her voice mail.
"No," she said.
Mills and McCartney divorced in 2008.
Morgan has said in the past that he has never hacked a voice mail and vigorously denied ever ordering phone hacking. Morgan said Thursday, "I have nothing further to add to the evidence I gave to the Leveson Inquiry."
The publisher of the defunct News of the World paid out hundreds of thousands of pounds Wednesday to settle lawsuits over phone hacking from celebrities and politicians, including former Tony Blair spokesman Alastair Campbell.
Piers Morgan is a former editor the News of the World, but left in 1995 -- about seven years before the Milly Dowler hacking -- and went to the Mirror, which he edited until 2004.
With Wednesday's settlements, News Group Newspapers has settled 59 of the 60 lawsuits against it.
But former child singing star Charlotte Church, who has testified publicly about the damage phone hacking did to her personal life, has refused to settle. Her case is expected to go to court as soon as this month.
In addition to the Leveson Inquiry, police are carrying out three separate investigations into elements of the scandal: phone hacking, e-mail hacking, and police bribery.
Two parliamentary committees are also investigating the scandal.
Both James and Rupert Murdoch, as well as senior executives at News International, have testified before British lawmakers examining allegations of wrongdoing.
There have been 13 arrests in connection with Operation Elveden, the Metropolitan Police investigation into bribery of police officers by journalists.
There have been 17 arrests in relation to Operation Weeting, the phone-hacking inquiry. Three people have been arrested in connection with both investigations.
No one has been charged.
CNN's Laura Smith-Spark and Jonathan Wald contributed to this story.