Met officers have met with the Crown Prosecution Service to discuss policy
Crown prosecutors have said Sir Ian Blair has misunderstood the law in making a call for celebrities caught on film taking drugs to stand trial.
The Metropolitan Police Commissioner had called for a change in policy by the Crown Prosecution Service (CPS).
He cited the case of model Kate Moss, who was not prosecuted as police were unable to prove what substance she had allegedly been filmed using.
The CPS said it pressed charges when offered sufficient police evidence.
Sir Ian supported an investigation into the claims against Kate Moss, citing the possible impact of such behaviour on "impressionable young people".
But she escaped prosecution because police were unable to prove what substance she was allegedly using in a west London recording studio.
"I was quite clear that I expressed my concern over the Kate Moss story, and we did that same investigation and we hit the same issue," Sir Ian said.
"And I think it's reasonable for a jury to be able to say, beyond reasonable doubt, 'I can see that behaviour, you convince me that you're taking talcum powder, because that's an unusual way to take it.'
But the Director of Public Prosecutions Sir Ken Macdonald QC, said: "If he is accurately quoted he appears to have completely misunderstood the law.
"The issue was not whether the white powder that Kate Moss was snorting was cocaine or talcum powder.
"The law required us to prove that it was either a class A drug or a class B drug. We could only base our case on one of these options.
"It was impossible for us to do this since cocaine - a class A drug - and amphetamine - a class B drug are both white powders."
He added: "Any suggestion that the CPS does not prosecute celebrities is completely untrue.
"We will prosecute when the police provide us with sufficient evidence to do so."