The head of Sky News said journalists have to consider breaking the law to "shed light" on wrongdoing as it emerged that media regulator Ofcom is launching an investigation into the hacking of private email accounts by the broadcaster.
Ofcom announced the investigation after the news channel admitted it had accessed the accounts of back-from-the-dead canoeist John Darwin and his wife Anne, as well as those of a paedophile.
The watchdog said it is investigating the "fairness and privacy issues" raised by the hacking.
John Ryley, head of Sky News, told the Leveson Inquiry into press standards that occasions where a journalist would break the law in pursuit of a story would be "very, very rare".
"Journalism is at times a tough business," he said. "And we need at times to shed light into wrongdoing. There may be an occasion. It would be very, very rare."
Darwin, 61, faked his own death in a canoeing accident in 2002 so his wife could claim hundreds of thousands of pounds from insurance policies and pension schemes.
The Darwins, from Seaton Carew, near Hartlepool, were jailed at Teesside Crown Court in 2008 for the swindle, which deceived the police, a coroner, financial institutions and even their sons, Mark and Anthony.
The broadcaster said evidence discovered by north of England correspondent Gerard Tubb was handed to police and used in the successful prosecution of Mrs Darwin, 60, for insurance and pension fraud.
The inquiry heard how Mr Tubb learned from a "source close to the prosecution" that an email account used by Mr Darwin was not going to be used as evidence.
Mr Ryley said Mr Tubb had accessed the account in June 2008 while working on a "court backgrounder" to be broadcast at the conclusion of the trial.
"He had been working on it for five to six months," Mr Ryley said.
"It became apparent that the email account would not be used by the prosecution.
"John Darwin had been using emails to go about his business in the five years that he had disappeared.
"Sources close to the prosecution made clear that they were not going to be following up on the emails."
Ten days after accessing the account, the findings were reported to the police.
The detail was "pivotal" in the case against the fraudsters, the inquiry heard.
Mr Ryley was also questioned about a Sky News' decision to access emails when following a story about a couple - Martin and Lianne Smith - who fled to Spain when police "took an interest" in Mr Smith, a "suspected paedophile".
Mr Smith, whose partner Lianne is accused of murdering their two children in a Spanish hotel, was extradited back to the UK but has since died in prison.
Mr Ryley was asked about the "justification" for the decision to allow a reporter to "access" to the emails of Mrs Smith - a former childcare worker with a local authority.
He said there were "reasonable grounds" for suspecting that if a local authority had "done more" it might have been able to find the family.
"It was a poignant story that we were interested in investigating," said Mr Ryley.
"If we'd been able to demonstrate that the local authority had in some way failed, then that might have led to a change in systems and procedures that would save the lives of other children of - ensure there was less harm in the future."
The hacking of the emails of the Smiths did not lead to the publication or broadcast of any material.
When the hacking incidents emerged less than three weeks ago, Sky News defended its actions as being in the public interest and said it amounted to "responsible journalism".
In a statement, it said: "We stand by these actions as editorially justified and in the public interest."
A Ofcom spokesman: "Ofcom is investigating the fairness and privacy issues raised by Sky News' statement that it had accessed without prior authorisation private email accounts during the course of its news investigations.
"We will make the outcome known in due course."
The Ofcom code states that "any infringement of privacy in programmes, or in connection with obtaining material included in programmes, must be warranted".
The regulator has a variety of potential sanctions for breaches of its code, depending on the severity. They range from a simple ticking off, through to a fine or the revocation of a licence in the most serious circumstances.
A spokeswoman for Sky News said today: "As the head of Sky News, John Ryley, said earlier this month, we stand by these actions as editorially justified.
"The Crown Prosecution Service acknowledges that there are rare occasions where it is justified for a journalist to commit an offence in the public interest.
"The Director of Public Prosecutions Keir Starmer told the Leveson inquiry that 'considerable public interest weight' is given to journalistic conduct which discloses that a criminal offence has been committed and/or concealed."