Prince Harry’s legal battle against a powerful British tabloid publisher is heating up -- with the Duke of Sussex returning to court to take them on over allegations of unlawful information-gathering ... along with a host of A-listers.

Harry arrived at London’s High Court on Monday looking confident in a black suit … acknowledging a few spectators as he headed inside.

Elton John and actors Elizabeth Hurley and Sadie Frost join Harry as part of the group, claiming Associated Newspapers Limited -- the publisher of the Daily Mail -- hired private investigators to place listening devices in homes and cars and record private calls. There are also claims of ANL engaging in impersonation and deception to access personal info.

ANL has vehemently denied these accusations, calling the claims “simply preposterous.”

The trial kicked off Monday and is expected to last nine weeks ... with Harry expected give evidence on Thursday.

The plaintiff's attorney, David Sherborne, wasn’t holding back punches ... telling the court there was a long-running culture inside Associated Newspapers to unlawfully dig up dirt “that wrecked the lives of so many.”

You'll recall ... Prince Harry and co. fired off their lawsuit against ANL in 2022 -- and since then, the two sides have sparred in court. In 2023, Harry won a legal judgment that found the publishers of the Daily Mirror engaging in “widespread and habitual” phone hacking.

Stay tuned.