Ministers have been accused of "fiddling the figures" after new statistics undermined claims made in a notorious Home Office press release.
The data, put out last year despite protests from NHS statisticians that it had not been fully checked, purported to show the number of knife crime victims treated by doctors in ten knife crime hotspots fell by 27%.
But newly published NHS data, issued after the statistics were verified, showed the actual fall between July and September was 18% and the annual decline was only 8%.
Shadow justice secretary Dominic Grieve said: "We now see the full extent of the government's fiddled figures. Ministers should stop spinning and take decisive action to deal with the scourge of knives."
Ministry of Justice figures also released on Thursday showed the number of offenders given jail terms for knife offences shot up by nearly a quarter. Figures for October to December showed average sentence lengths also soared, by 38% to 26 weeks.
Justice Secretary Jack Straw said the figures showed the Government's "tough" approach was working. He said: "This sends a very important message to knife criminals and would-be knife carriers to pack it in."
But critics pointed out that fewer than one in five knife criminals was given an immediate jail term.
In the last three months of last year the courts dealt with 6,704 knife offenders and only 1,386 were given an immediate custodial sentence. And more criminals - a total of 1,706 - were given cautions than put behind bars.
The figures also showed a modest fall of 2% in the total number of knife offences committed across England and Wales in the period compared with a year earlier. But in the ten Tackling Knives Action Plan (TKAP) areas - which were given millions of pounds of Government money - there were only 43 fewer offences committed - a fall of less than 1%.
The sentencing changes follow a Court of Appeal ruling last May which said knife carriers should be given longer jail terms. Sentencing guidelines for the courts, which were changed following the judgment, set the basic sentence for knife possession by adults at 12 weeks in prison.
