Knife amnesties have no real impact on reducing knife crime, according to a Metropolitan Police report.
More than 100,000 knives were handed in to forces this summer
An eight-week Met Police amnesty over the summer showed that, after a slight dip in knife-related crime, levels were back to normal within weeks, it said.
Pre-amnesty there were an average of 34.9 daily knife offences in London, dropping to 31 in the amnesty. Six weeks later, that figure was 34.2.
The report was obtained by the BBC under the Freedom of Information act.
More than 100,000 knives were handed into police forces in England, Scotland and Wales this summer during a five-week nationwide campaign and the amnesty was declared a success by ministers.
But an analysis of the concurrent but longer eight-week amnesty by the Metropolitan Police - between May and July - appears to question the effectiveness of the operation.
The report said "no changing trends" had been visible during the first five weeks of the operation from 15 May.
For the final three weeks of the operation and in the following weeks "a small downward trend appeared".
"In late August 2006 - six weeks after the operation ends - offence levels tend to return to pre-operation values", the report adds.
A spokesman for the Metropolitan Police said the force always recognised that a knife amnesty on its own was of limited effectiveness in tackling knife crime.
Amnesties needed to be used in conjunction with other initiatives, the spokesman added.