By Sam Tobin

LONDON (Reuters) -Prince Harry should receive a maximum of just 500 pounds ($637) in damages for one admitted instance of unlawful information gathering, lawyers representing a British tabloid newspaper group told London's High Court on Tuesday.

Harry, King Charles' younger son, is one of more than 100 people suing Mirror Group Newspapers (MGN), the publisher of the Daily Mirror, Sunday Mirror and Sunday People, over allegations of phone-hacking and unlawful information gathering.

Their lawyers allege unlawful activity was "widespread" at all three MGN newspapers between 1991 and 2011.

The fifth-in-line to the throne spent a day-and-a-half in the witness box earlier this month, being grilled over allegations he had been unlawfully targeted by MGN titles for 15 years from 1996, when he was a child.

His cross-examination, when he became the first senior British royal to appear in a witness box for more than 130 years, began with an apology from MGN's lawyer Andrew Green for one instance of unlawful information gathering.

MGN admitted at the start of the trial in May, which concludes this week, that on one occasion a private investigator had been engaged to unlawfully gather evidence about Harry at a London nightclub in 2004, for which it "unreservedly apologises".

However, the publisher argued in court filings released on Tuesday that Harry "has failed to identify any evidence of voicemail interception against him, nor any other evidence of unlawful information gathering in respect of his private information", apart from the one incident it has admitted.

"The Duke of Sussex should be awarded a maximum of 500 pounds given the single invoice naming him concerns enquiries on an isolated occasion and the small sum on the invoice - 75 pounds - suggests enquiries were limited," Green said.

(Reporting by Sam Tobin; editing by Sarah Young)