Rupert Murdoch is not legally obliged to hand over evidence of wrongdoing in his newspapers to the police, contrary to claims he made in a letter to News International staff, a leading human rights lawyer has said.

Geoffrey Robertson has said that Murdoch's letter in relation to this issue "is full of errors, both of law and history".

He added that the media baron was "ill-advisedly and unethically throwing away the shield that parliament gave to journalists in 1984 so they could protect their sources".

On Friday, Murdoch told staff that his internal investigations unit, which had already handed over evidence that has led to the arrest of nine Sun journalists in the past month, would continue to disclose material to the police because the company was "obligated" to do so.

"We will turn over every piece of evidence we find – not just because we are obligated to but because it is the right thing to do," Murdoch's email said.

He said he would "continue to ensure that all appropriate steps are taken to protect legitimate journalistic privilege and sources" but warned he "cannot protect people who have paid public officials".

Robertson said: "Apparently, he thinks it is right to hand over confidential source material – including the names of whistleblowers – to police without them even asking. This is a breach of the most fundamental ethic that journalists must not betray their source and there is no law that requires it.

"On the contrary, the 1984 Police and Criminal Evidence Act defines confidential journalistic material as 'excluded material', which police cannot seize at all, other than in a few cases such as official secrets, when they have to get an order from a circuit judge."

In 1984, a delegation from the press, led by Lord MacGregor and including James Evans, the then-solicitor for Times Newspapers, and Geoffrey Robertson saw the home secretary in order to ensure that the 1984 Police Act provided a special procedure whereby confidential journalistic material could only be obtained by police applying to a circuit judge with proof that the public interest required them to have it. This doesn't apply, however, if a voluntarily give their documents to the police or invite police into their offices to take what they want."

News International insiders have expressed concern that Murdoch's letter to staff means that his policy of voluntarily handing over documents to the authorities has not changed, and it will continue to disclose sources in breach of the code of practice for journalists.

Robertson added: "What is so unattractive about Mr Murdoch's behaviour is that he is handing over journalists without ever asking them, or their editors, or their executives who must have signed off on the payments, what they were doing and whether they were genuinely pursuing a public interest story. Any significant payment must have been approved by executives, and News Corp does not appear to have turned them over."

He added: "The real danger of this behaviour is that it is a blow to investigative journalism, which depends on the cultivation of sources. "Whistleblowers will be much less likely to come forward, however much they trust the journalist, if they fear that his proprietor may turn them over to police."

"Everyone seems to have forgotten that over 200 years ago John Wilkes went to prison to stop government agents getting hold of his journalistic material without a specific warrant. He sued the government and won a great constitutional case. He would be turning in his grave."