Metropolitan police commissioner Sir John Stephens has admitted that new cannabis laws had caused "a massive amount of muddle".
Sir John Stephens says he made 'a big mistake' over a softly-softly approach
Sir John told LBC radio that police needed to give a clear message that the drug was still illegal.
"The use and possession of cannabis is still against the law," he said.
But from 29 January, cannabis is to be downgraded from class B to class C, and fewer users will face arrest if caught in possession.
Police will be more likely to confiscate the drugs than make arrests, and officers will stop targeting those using cannabis in their own home.
Against the law
Sir John told listeners: "I think there is a massive amount of muddle about where we are on cannabis."
"The position is that the use and possession of cannabis is still against the law in this country," he said.
NOT LEGAL YET
Cannabis will become a class C drug from 29 January
Police will not arrest most users, but may confiscate drugs
Under-18s and those using near schools will still be arrested
"I think that needs to be made absolutely clear."
He said he regretted elements of the "softly-softly" Brixton pilot scheme, which he said had ignored "the nature of Brixton" and caused the borough to become "a goldfish bowl".
"I think I made a big mistake," he said.
The police chief also said he was against the legalisation of drugs.
"I have been against it ever since I went and saw the experiment in Amsterdam in 1986 where there had been a legalisation of not just cannabis but of hard drugs."
The Dutch trial had led to "a massive surge of vigilantism", murder and arson, he warned.
"I could not stand by as a police officer and watch that happen."