Society needs journalists who are prepared to break the law in order to serve the public interest, argued the former director of public prosecutions Sir Ken Macdonald last night.

Speaking at a debate at City University on the the News of the World phone-hacking case and the lengths to which reporters can go to get information, MacDonald said: “There are bound to be cases where journalists will want to break the law, and for good reason (…) We do want journalists to break the rules.”

Macdonald did not condone the phone-hacking at NotW, and stressed that it was only under certain public interest circumstances that journalists might be forgiven for breaking the law.

He was joined by key players in the phone hacking scandal: Nick Davies of the Guardian, ex-News of the World journalist Paul McMullan and defamation lawyer Mark Lewis, as well as Max Mosley, Roy Greenslade and libel barrister Caldecott QC.

Mark Lewis, who is currently suing the Metropolitan Police and the Press Complaints Commission for libel, echoed Macdonald, saying that in certain circumstances illegal activity is acceptable.

“If you know something is of public interest then you can use certain methods to corroborate it,” he said. However, he stressed that these methods should not be used to obtain a story.

Macdonald also cautioned against increasing privacy laws, warning it could create a “contagion of caution” among newspapers, and pointed out that a culture of deference has developed in France due to its strict privacy rules.

However, Macdonald conceded that it is nearly impossible to define what is and isn’t in the public interest.

As former Daily Miror editor and journalism professor Greenslade pointed out, “the public interest for the Guardian’s audience is very different to the public interest of the News of the World readers.

“There is no easy way of drafting a public interest definition that would give journalists clear guidance on what they should and shouldn’t publish.”

More from Journalism.co.uk:

Former News of the World journalist defends phone-hacking at lively debate

PCC claimes it did respond to Dispatches with phone-hacking statement

Phone-hacking on Dispatches: a good documentary but not enough new evidence