Britain's most senior prosecutor has launched a robust defence of journalists who break the law pursuing investigations that have a genuine public interest. Legal guidelines had been drafted, he said, to protect reporters.

Keir Starmer, the director of public prosecutions (DPP), insisted it "would be very unhealthy if you had a situation where a journalist felt that they needed to go to their lawyer before they pursued any lead or asked any question".

In an interview with the Guardian, Starmer said: "We've got to recognise that in the course of journalism, journalists will rub up against the criminal law and that is why, in our guidelines, we took the approach that we would assess where there was evidence of a criminal offence, whether the public interest in what the journalist was trying to achieve outweighed the overall criminality."

Starmer spoke at the end of another week in which the furore over the leaks from the whistleblower Edward Snowden has reverberated around Westminster. One backbench Tory MP has called for the Metropolitan police to investigate the Guardian for publishing stories about GCHQ's mass surveillance programmes.

Without mentioning Snowden, Starmer made clear he drafted guidelines specifically to allow journalists to pursue difficult stories without fear of prosecution, so long as a public interest threshold had been reached. He acknowledged there were "potentially many offences that journalists could commit in the course of their business" but said guidelines were there to offer them protection.

"There are lots of examples of journalists who, on the face of it, may have broken the criminal law but have obviously pursued a greater good in doing so," said Starmer, who will step down as DPP at the end of the month.

"That is why we wanted to issue guidelines, and our approach is very clear: first we look to see if an offence has been committed; well, if not, that's obviously the end of it. If an offence has been committed, we then say: did the public interest in what the journalist was trying to achieve outweigh the overall criminality, taking into account the nature of the lead, how much information there was, what they were trying to uncover etc?"

He added: "Defining of the public interest is always very, very difficult. We did go through a consultation exercise on that and I think we've got it about right."

Starmer joined lawyers, academics and senior members of the intelligence community who have said the laws governing Britain's intelligence agencies need to be reviewed in the light of Snowden's revelations about mass surveillance programmes run by GCHQ and its US counterpart, the National Security Agency.

The Intelligence Services Act was passed in 1994, and the Regulation of Investigatory Powers Act in 2000 – long before some recent technological capabilities became available. "I think there's a growing recognition that the legislation in place needs to be looked at again to see whether it works well in the current environment … some of the old laws should be looked at again. I think most people accept that it is necessary to have some surveillance in a democratic society. I think most people accept that it's important to have limits and clear safeguards on that."

The freedom of speech campaign group Article 19 described calls for the Guardian to be prosecuted as deeply concerning because the Snowden disclosures had kickstarted "a much-needed public debate about blanket surveillance".

The New-York-based Committee to Protect Journalists said Scotland Yard's inquiry was "chilling."

Earlier this week the home affairs select committee said it would investigate the Guardian's publication of stories about mass surveillance as part of a wider inquiry into counter-terrorism.

Nick Clegg, the deputy prime minister, has said the extent of state surveillance needs to be properly debated. The business secretary, Vince Cable, said last week the Guardian had performed a considerable public service by publishing stories about surveillance. He said the Guardian had been "entirely correct and right" and "courageous".

Starmer's predecessor as DPP, Lord Macdonald, earlier this week accused the head of MI5 of using "foolish, self-serving rhetoric" to resist legitimate calls for Britain's intelligence agencies to face more scrutiny.

Macdonald said: "It seems very obvious that when it comes to surveillance and techniques of domestic spying, the law should be the master of technology. Anything else risks a spiralling out of control, an increasing subservience of democracy to the unaccountability of security power."

Sun journalist charged

A journalist from the Sun has been charged with offences relating to the alleged theft of Labour MP Siobhain McDonagh's mobile phone.

Nick Parker, the Sun's chief foreign correspondent, is accused of one offence of receiving stolen goods and one offence of unauthorised access to computer material. A second man, Michael Ankers, 29, from south-west London, was charged with theft, over a mobile phone allegedly stolen from McDonagh in 2010.

They will appear at Westminster magistrates' court on 6 November.