A High Court ruling has been hailed as a major step in reinforcing the rights of solicitors.
Lord Justice Balcombe and Mr Justice Buxton last week found a Metropolitan Police detective guilty of contempt for obstructing London firm JD Spicer & Co in its attempt to trace possible alibi evidence for a client facing a murder charge.
After a homeless man was found dead two others were charged with his murder. But one of them asked Spicers to trace other homeless people, who he said could provide him with an alibi.
However, Lord Justice Balcombe said police refused to allow a private investigator to show hostel staff a photograph of the murder suspect. Det supt Derick Dale also warned that "appropriate action" would be taken if it was shown.
However, the court held that this threat should be classed as contempt of court because it amounted to interference with witnesses or potential witnesses. No steps were taken against Det Supt Dale because he had already undertaken not to interfere with Spicers' inquiries in the future.
A Spicers spokesman says: "All our firm was trying to do was prepare the evidence for our client's defence.
"It is obviously important that solicitors should be able to go about their job without being threatened with arrest for obstructing the police in the execution of their duty."