Murderers and terrorists could walk free from prison because of a "potentially disastrous" ruling by the Law Lords, a senior police officer said.
Defendants in criminal trials will have a legal right to know the identity of the witnesses testifying anonymously against them.
John Yates, assistant commissioner of the Metropolitan Police, said the ruling could lead to a series of appeals from criminals who have been convicted by evidence given anonymously in court.
His comments come after Iain Davis' double murder conviction was quashed by the Law Lords earlier this week.
Police now believe that up to 40 murderers and serious criminals in London alone could now appeal and would walk free if the witnesses refuse to reveal their identity in a retrial.
John Yates, assistant commissioner of the Metropolitan Police, told a newspaper: "This is potentially disastrous.
"A lot of good work being undone, and this will play out so badly in terms of those we are trying to reach out to in communities. It almost feels like we have broken our word.
"To see clearly guilty people walking free is just awful.
"Special measures are only used in the most extreme cases, which means these are our most dangerous criminals, people who have been jailed for up to 40 years. And they could be walking free."
Davis was convicted at the Old Bailey of the New Year's Day 2002 killing of two men in a shooting at a flat in Hackney, east London. Seven witnesses at the trial in May 2004 claimed to be in fear for their lives if it became known that they had given evidence against Davis.
