News International cannot rule out the possibility that News of the World staff continued hacking phones even after one of the paper's reporters was jailed for the illegal practice, the Leveson Inquiry has heard.
But the publisher questioned claims that as many as 28 of the Sunday tabloid's journalists commissioned private investigator Glenn Mulcaire to intercept voicemail messages.
It emerged that the family of murdered schoolgirl Milly Dowler, whose phone was hacked by the News of the World after she vanished in 2002, will be the first witnesses to give evidence to the inquiry into press standards next week.
A total of 21 people who have complained about press intrusion - including actor Hugh Grant, the parents of missing Madeleine McCann and former Formula 1 boss Max Mosley - are expected to appear before the hearings.
Former information commissioner Richard Thomas will tell the inquiry on December 1 about his investigation into the unlawful trade in confidential personal information.
Ex-Number 10 communications director Alastair Campbell will also give evidence. He is expected to focus on the relationship between national newspapers and politicians.
Rhodri Davies QC, counsel for News International, told the inquiry that "lessons were learned" when News of the World royal editor, Clive Goodman, was jailed in 2007 for hacking royal aides' phones.
But he admitted: "I am not going to give any guarantees that there was no phone hacking by or for the News of the World after 2007."
Police believe that illegal voicemail interception at News International had begun by 2002 and continued until at least 2009, the inquiry sitting at the Royal Courts of Justice in London has heard.
Mulcaire's notebooks, which were seized by detectives in 2006, suggest that at least 27 of the publisher's employees other than Goodman commissioned him to hack phones, the hearing was told.