A debate organised by the NUJ at the House of Commons last night on protecting journalists' sources highlighted the new difficulties thrown up by so much information being held on computers.
Gavin Millar QC said it was a "grey area" who owned the material that the courts might demand should be handed over to identify a source - the journalist or their employer.
He said in the days of notebooks, employers could argue that material wanted by police to identify a source was not on their premises but was in the possession of the journalist.
Michelle Stanistreet, NUJ general secretary, said in some past cases the union had arranged for material identifying sources to be sent abroad so it was beyond the jurisdiction of the courts.
But this is far more difficult in the electronic age where emails, notes and leaked information could be stored on computers owned by publishers and broadcasters.
BBC journalist Peter Taylor said it was yet to be tested who owned source material - the journalist or his employer - although he had always been backed by the BBC when he refused to provide information that could have named sources.
Stanistreet said the decision by the News Corp Management and Standards Committee to hand over emails and other material gathered by Sun reporters to police was unprecedented and "a betrayal of sources".
She claimed: "One overriding principle of all journalists is the protection of sources".
- The NUJ, Sky News, ITN and the BBC will be in court next week for a judicial review of production orders requiring unbroadcast film of the Dale Farm evictions being handed over to police.