Harry, the younger son of King Charles, and six others including singer Elton John have accused Associated Newspapers' tabloids of being involved in widespread unlawful information gathering including phone-hacking dating back 30 years.
Associated, which also publishes the Mail on Sunday, has denied any wrongdoing.
The trial has already heard evidence from Harry and the other claimants as well as numerous senior current and former journalists and staff at Associated.
On Monday private investigator Gavin Burrows, whose testimony could decide the outcome, said the lawsuit "was based on a pack of lies".
In August 2021, the claimants' lawyers say Burrows signed a witness statement in which he stated he had "targeted hundreds, possibly thousands of people" for Associated, from tapping landlines and hacking voicemails to obtaining information by deception.
Those allegations help form a substantial part of the claimants' case.
Burrows later told Associated's legal team he had never made this statement and that his signature had been faked, telling the court he had first heard about the allegations attributed to him by reading a newspaper report.
"This statement has nothing to do with me," Burrows - who gave evidence from an undisclosed overseas location as he says he and his family have received threats - told the court by videolink.
"You have got to explain to your claimants how you have been conned," he said during testy exchanges with their lawyer David Sherborne who was given permission to treat his own witness as "hostile".
"This thing is based on a pack of lies."
Associated have cast the whole case as manufactured and funded by opponents of the press such as the late motor racing boss and privacy campaigner Max Mosley, and that a "research team" assisting the claimants' lawyers had paid witnesses to provide evidence.
Sherborne accused Burrows of lying, suggesting he had only decided to change his evidence after he fell out with one of the research team, Graham Johnson, a journalist who was convicted of phone-hacking and now writes about tabloids' unlawful activities.
Johnson has previously told the court that Burrows had agreed to a book deal and to help with documentaries for which he had been paid STG75,000 ($A142,971), and that their relationship fell apart in early 2022.
Burrows said he had no knowledge that he would be used in the Associated litigation until January 2023 when he became "absolutely furious" that his name was being linked.
He said he approached the publisher because he thought one of the claimants, racism campaigner Doreen Lawrence, was being conned.
"The whole thing is a thing of fiction," he said.
He told the court he had never worked for or been paid by Associated.
Burrows is the last witness at the trial, which began in January, with closing submissions due later this month.
