It is “fundamentally unfair to tarnish journalists” with “career and reputation-destroying” allegations of involvement in unlawful information gathering, a lawyer for the publisher of the Mirror has told the High Court.

Mirror Group Newspapers (MGN) is being sued by the Duke of Sussex and other individuals over claims reporters at it titles – the Daily and Sunday Mirror and Sunday People – were linked to methods including phone hacking, so-called “blagging” or gaining information by deception, and the use of private investigators for unlawful activities.

Continuing his closing arguments on Thursday, MGN’s barrister, Andrew Green KC, told the trial in London that it was “inappropriate” for certain allegations, including of criminal acts, to “simply be bandied around like confetti”.

The barrister challenged arguments by lawyers for Harry and others that a judge should rule that journalists across all three of its titles had engaged in “habitual and widespread” unlawful activity from 1991 to 2011.

Mr Green said there was “simply no evidence” to support such a finding over alleged phone hacking, with the claimants’ lawyers’ arguments being “an absolute non-starter”.

He told judge Mr Justice Fancourt that he was being invited to find that “systemic” criminal activity had taken place across MGN titles over a number of years, adding that these were “incredibly serious allegations to be making and it should not be made lightly”.

Mr Green said there was “no evidence” of phone hacking beyond a period covered by a previous 2015 High Court ruling covering similar claims against MGN.

He said it was “fundamentally wrong” for the judge to make “generic” findings that MGN journalists had engaged in unlawful activity, which he said the claimants’ lawyers asked for in relation to “an enormous number of people”.

“We say that’s quite outrageous… and should not be permitted,” Mr Green said, adding: “They are findings… that cannot be made by the court.”

These were “career and reputation-destroying allegations which are potentially liable to get the police interested”, Mr Green also said.

“They are simply bandied around like confetti,” he added, noting that one ex-journalist witness in the trial – Tom Worden – was now a practising barrister.

“You are being invited to in effect destroy his career,” Mr Green told the judge.

Mr Green later said that Harry’s case included an allegation that former royal correspondent Jane Kerr was a “prolific user of private investigators”.

The barrister told the court: “It simply has not been established that she has got a history of unlawful information gathering.”

He said such a claim was a “quite unfair, indeed reprehensible slur, of Ms Kerr”.

“These are career-destroying allegations… against Ms Kerr,” he added.

Mr Green said there was “nothing to suggest” she was involved in unlawful information gathering in the evidence.

“This is just fundamentally unfair to tarnish journalists in that way,” Mr Green said.

The barrister gave another example of former Daily Mirror journalist and news editor Anthony Harwood, who wrote one of the articles the duke complains about, who “occasionally appears to have instructed a third party supplier to find an ex-directory phone number”.

Mr Green said there was “absolutely no basis for the suggestion… that he did that for the purposes of voicemail interception”, adding that such a request could be for “perfectly legitimate” journalistic purposes and could be achieved “by perfectly lawful means”.

The barrister said that even if a “civil wrong” was involved it was “about as low level (unlawful information gathering) as you can imagine”.

Mr Green also said the claimants had made “hopeless” and “arguable” claims that the judge should conclude that “every invoice or payment” relating to third party suppliers named in the case indicated unlawful activity.

MGN’s lawyer also rejected “criticism” of his cross-examination of witnesses earlier in the trial, saying that given the “very serious” nature of allegations made it was “perfectly legitimate to cross-examine as to their reliability”.

He added that the “suggestion that I shouldn’t have done that is completely wrong”.

Harry’s case is being heard alongside similar claims by actor Michael Turner, who is known professionally as Michael Le Vell and best known for playing Kevin Webster in Coronation Street, actress Nikki Sanderson and comedian Paul Whitehouse’s ex-wife Fiona Wightman.

The findings made by Mr Justice Fancourt in relation to the four will be used to determine the outcome of dozens of claims brought by others against MGN, including actor Ricky Tomlinson, the estate of the late singer George Michael, ex-footballer and television presenter Ian Wright and Girls Aloud singer Cheryl.

MGN is largely contesting the claims and denies that any of the articles complained of resulted from phone hacking, while contending that the vast majority did not arise from any other unlawful activity.

The publisher has made a limited number of admissions of unlawful activity in relation to the duke, Ms Sanderson and Ms Wightman, for which the publisher has apologised and accepted they will be entitled to some damages, but denies the majority of their claims and Mr Turner’s entire case.

The trial is due to conclude on Friday and Mr Justice Fancourt will deliver his ruling at a later date.