Heather Mills has alleged that a senior Mirror Group journalist admitted hacking voicemails left for her by her then-boyfriend Sir Paul McCartney.
Ms Mills told BBC Newsnight that after Sir Paul left the voicemail in 2001, the journalist rang her quoting parts of the recording.
When challenged about how they knew what had been said, Ms Mills said they admitted the message had been hacked.
Parent group Trinity Mirror says all its journalists work within the law.
Mirror Group Newspapers is part of Trinity Mirror plc, which publishes over 260 titles including the Daily and Sunday Mirror, Daily Record and People.
Trinity Mirror responded to the allegation by saying: “Our position is clear. All our journalists work within the criminal law and the PCC [Press Complaints Commission] code of conduct.”
Ms Mills told Newsnight that in early 2001 she had had a row with former Beatle Sir Paul, who later left a conciliatory message on her voicemail while she was away in India.
According to Ms Mills, afterwards a senior Mirror Group Newspapers journalist rang her and “started quoting verbatim the messages from my machine”.
Ms Mills said she challenged the journalist saying: “You’ve obviously hacked my phone and if you do anything with this story… I’ll go to the police.”
She said they responded: “OK, OK, yeah we did hear it on your voice messages, I won’t run it.”
The journalist whom Ms Mills said contacted her is not CNN presenter Piers Morgan, who was the editor of the Daily Mirror at the time.
However, the message in question appears to be the same as one which Mr Morgan later admitted to having listened to.
In a 2006 article in the Daily Mail, Mr Morgan referred to having heard a recorded message which Sir Paul had left for Ms Mills.
“At one stage I was played a tape of a message Paul had left for Heather on her mobile phone,” he wrote.
“It was heartbreaking,” Mr Morgan wrote. “The couple had clearly had a tiff, Heather had fled to India, and Paul was pleading with her to come back. He sounded lonely, miserable and desperate, and even sang ‘We Can Work It Out’ into the answer phone.”