The Sunday People may have targeted the mobile phone of murdered schoolgirl Milly Dowler, court papers suggest.
The tabloid newspaper is alleged to have hired private detectives to target the Dowler family in 2002.
This has emerged in a civil case at the High Court brought by 71 claimants, including Prince Harry and the author and actor David Walliams, against Mirror Group Newspapers.
The current publishers of the People, Reach Plc, have declined to comment.
A defence to the allegations has not yet been submitted and the case continues.
In 2011, another tabloid, the News of World, was closed after revelations it hacked Milly's phone.
In court papers presented last November and seen by the website Byline Investigates and now the BBC, it is alleged the Sunday People hired the private investigation firm Starbase to target the Dowler family 18 years ago.
At that time Milly, 13, was missing and her disappearance was a major national news story.
The claimants allege the Dowlers were also put under surveillance and that Starbase was involved in phone hacking and other unlawful information gathering.
They point to an unusually large invoice from Starbase for work at that time and an article that appeared in the People 10 days after Milly's disappearance as evidence of unlawful information gathering.
Press standards campaigner Prof Brian Cathcart said of the latest allegations against the People: "This extremely serious allegation raises the shocking possibility that a second newspaper was illegally hacking Milly's phone.
"If it is true, we need to know who authorised it and how many people knew. The Metropolitan Police refused to take the matter in hand, and the civil courts are not the place to get to the bottom of it."
The revelation Milly's phone was hacked by the News of the World caused a wave of public revulsion and transformed phone hacking from a story about celebrities' privacy into a full blown national scandal.
At the height of the phone-hacking scandal, Rupert Murdoch shut the 168-year-old paper and then Prime Minister David Cameron commissioned the Leveson Inquiry into the ethics and culture of the press.
In 2015, Mr Justice Mann ruled phone hacking had been widespread and frequent for a decade at Trinity Mirror, the then owners of the Daily and Sunday Mirror and the People.
Neil Wallis was the editor of the People in 2002. There is no suggestion he commissioned the work allegedly carried out by Starbase.
He went on to become the deputy editor of the News of the World and was tried at the Old Bailey and acquitted in 2015 of conspiracy to hack voicemails during his time as deputy editor of the defunct tabloid.
He denied any knowledge of the illegal practice and described the prosecution as "politically motivated".
In 2014, his boss at the News of the World, Andy Coulson, who went on to become David Cameron's director of communications, was convicted of the same offence and jailed for 18 months.
Mr Wallis left journalism and took up PR work in 2009, including a contract with the Metropolitan Police.
The then Met Commissioner Sir Paul Stephenson resigned in 2011 after facing questions from MPs about links between the force and the News of the World.