The Metropolitan Police Commissioner Sir Ian Blair has admitted that there is "almost no public faith" in crime figures.
The New Scotland Yard head said the way crimes are categorised has been changed "so frequently they are bewildering".
He suggested that British authorities adopt the same system as in New York where crimes are recorded in a more straightforward way.
Speaking at the second Colin Cramphorn memorial lecture at the Policy Exchange last night, Sir Ian said the media and politicians have also weakened confidence in crime statistics.
He said: "Few question the crime figures in New York. Residents largely accept that their city is safer than it was.
"And that is because New York has not fiddled about with how they collect crime statistics in the way the UK has."
Sir Ian said many people would be surprised at how many crimes are recorded and said, for example, that gun enabled crime until recently included attacks with CS gas.
He mocked a definition of knife crime provided for police which includes an exhaustive list of weapons such as machetes, axes, crossbows, darts and needles.
He said: "...then is added, in a rather Monty Python way and I quote again, 'this list is not meant to be exhaustive', presumably in case someone is stabbed to death with a cocktail stick."
Sir Ian called for crime counting rules to be revised and simplified to make figures simpler and more credible.
Sir Ian said a similar transformation to New York policing has been taking place in London but in a "rather unstated British way".
He pointed to recent crime figures which show almost every category of crime is in decline in the capital.
Sir Ian said the Metropolitan Police welcomed plans by new Tory Mayor Boris Johnson to introduce New York-style crime mapping.
He said: "But it would help if the data that made up the map were of a sort that fully makes sense."
Sir Ian also spoke about his blossoming relationship with the new City Hall administration which has made gang and knife crime one of its top priorities.
But he rejected calls by Mr Johnson for the Home Office to examine whether the Met should keep its national responsibilities such as counter-terrorism.
Mr Johnson has said that if such responsibilities were hived off Londoners would be able to have a Commissioner who is directly politically accountable.
Some City Hall watchers regard this as the first step in allowing the Mayor to effectively appoint the Commissioner.
Sir Ian said giving up London's counter-terrorism lead would be a "poor bargain".
He said that creating separate national units could result in unhelpful inter-force rivalries like those in the United States.
He said: "I have made clear my view that the Mets' primacy for counter-terrorism allows us to protect London and the rest of Britain, not only in London but, in collaboration with other forces, across the nation and, in part, across the world.
"There has scarcely been a terrorism plot affecting the UK since 2000 that did not emerge from, run through or target London.
"Losing Met primacy in this field would diminish UK counter-terrorism capability, as well as risking bringing in its wake the disadvantages I have described in some parts of the US."
Speaking after the lecture, Kit Malthouse, who is deputy chairman of the Metropolitan Police Authority, said early talks between Sir Ian and the Mayor had been tentative.
He said: "It has been a delicate ballet around some of these issues like porcupines attempting to mate."