The Duke of Sussex, 41, and six other claimants are suing Associated Newspapers for alleged widespread unlawful activities ranging from hacking voicemail messages, bugging landlines and obtaining private information by deception over more than two decades from the early 1990s.
The publisher rejects their case as being "preposterous smears".
During almost 10 weeks of argument, judge Matthew Nicklin has heard evidence from the claimants as well as numerous current and former senior journalists from Associated, which also publishes the Mail on Sunday.
David Sherborne, the lawyer for Harry and the other claimants, said there was a culture at Associated's titles where its journalists would use private investigators to carry out unlawful activities on their behalf.
"Any finding of unlawful activity is a disaster," Sherborne said in his concluding remarks.
The publisher's lawyer Antony White argued that there was no evidence to back up the allegations, the claimants' witnesses were unreliable and the case against the papers was scattergun and part of a conspiracy by people with a grudge against the press.
"The remaining task is, of course, now mine," Nicklin said at the end of the trial.
"Judgment will take some time. After a short break over Easter ... I will be working on the case and the judgment effectively full-time ... so I won't be doing anything else ... and I will be toiling away on the judgment."
