Private detectives were hired by the Sunday People newspaper to target the family of the murdered schoolgirl Milly Dowler in 2002, it has been alleged.

The claims are made in legal papers lodged as part of a civil action being taken by a range of claimants, including Prince Harry.

Private investigators at a company, Starbase, are alleged to have been hired by executives at the newspaper as police tried to find the missing 13-year-old in March 2002.

One of the Sunday People’s rivals, the News of the World, closed in 2011 as its owner, Rupert Murdoch, sought to limit the political and commercial fallout from the phone-hacking affair and the revelation that an investigator working for it had hacked the schoolgirl’s phone.

But the new legal documents, lodged in the high court in November and first reported on by Byline Investigates, also allege that the Sunday People placed the Dowler family under “unlawful” surveillance before later publishing an article 10 days after the teenager disappeared.

Reach plc, the publisher of the Daily Mirror, Sunday Mirror and Sunday People, declined to comment.

Neil Wallis was the editor of the Sunday People between 1998 and 2003 before going on to become the deputy editor of the News of the World. There is no suggestion that Wallis, who has also been approached for comment, was involved in engaging Starbase.

He walked free from court in July 2015 after being cleared of conspiracy to hack phones at the News of the World and claimed his prosecution had been politically motivated.

Andy Coulson, the former Downing Street adviser to David Cameron, was sentenced the year before for 18 months for plotting to hack phones while he was in charge of the News of the World.

Press reform campaigners have said the allegations in the civil action raise the possibility that a second newspaper illegally hacked Dowler’s phone.

Prof Brian Cathcart, the journalism professor and co-founder of the Hacked Off campaign group, told the BBC: “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.”

Nathan Sparkes, policy director of the Hacked Off campaign group, said if the new allegations are true “then the Sunday People’s owners, Reach plc, will have serious questions to answer about why only now this is coming to light.

“The government will face accusations of complicity in any cover-up, having cancelled the only means of exposing the true of extent of phone hacking and other illegality at national newspapers: Leveson Part Two.

“Almost 18 years after Milly’s death, there is still so much we don’t know about the activities of the newspapers who targeted her phone for stories at the time and their relationship with the police, who knew that the News of the World hacked Milly’s phone back in 2002 but failed to act. Until the Government launch the Leveson Part Two Public Inquiry, we never will.”

Prince Harry filed legal proceedings against the Reach plc subsidiary MGN Ltd last October in an escalation of his war with sections of the British newspaper industry. He and his brother, Prince William, were at the centre of a series of hacking allegations after it emerged in the early 2000s that tabloid journalists were routinely accessing public figures’ voicemails to find stories.