Nearly one in three children admit to having carried a knife or a gun, a Youth Justice Board survey has found.

Seventeen per cent of 11- to 16-year-olds reported carrying a penknife and 15 per cent said they had carried a ball-bearing gun in the past year. One in 20 boasted of having carried a flick knife.

Almost half of the boys and one in six girls admitted carrying a knife or a gun. One in 20 said they had carried an airgun; 3 per cent had carried a "real" gun.

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However, the survey of 4,750 young people by the pollsters Ipsos MORI found significantly fewer young people were carrying weapons compared with 2005. Overall, just under a quarter of children admitted offending in the past 12 months, down from 27 per cent in 2005.

The findings were issued as the killer of the Harry Potter actor Rob Knox was given four life sentences. Karl Bishop was told he would be imprisoned for at least 20 years for the knife attack on Mr Knox, 18, and four of his friends.

Chris Grayling, the shadow Home Secretary, said: "These findings are absolutely shocking. Ministers tell us we are wrong to talk about a broken society. What clearer evidence do they need that Gordon Brown's Britain is going badly wrong?"

Chris Huhne, the Liberal Democrat home affairs spokesman, said ministers had failed to get a grip on knife culture. "It is a serious failing of our society that our young people feel the need to arm themselves for protection.

"Instead of posturing on penalties, Labour and the Tories should focus on what works to cut knife-carrying – increasing the likelihood of getting caught. This means hot-spot policing, intelligence-led stop and search, and restorative justice that brings offenders face to face with their own victims."

The Mayor of London, Boris Johnson, said yesterday that the judiciary was failing to tackle knife crime. "I am worried that the criminal justice system is not fit for purpose and is too often letting us down," he told the London Crimestoppers annual dinner. "I am worried that where someone is convicted of a knife crime on too few occasions do we get the sentence we need."

The survey found young people who broke the law commonly did so because they were bored. Alcohol and peer pressure were also blamed.

Fare-dodging was the most common offence, followed by shoplifting, minor assault, property damage and graffiti.

One in five young people said they had sent a text message threatening someone. One in six said they had been involved in "happy slapping" by using their phone to film a prank or assault.

There was an increase in the proportion of young people caught by police. Half of those who admitted committing crimes said they had been caught, compared with 29 per cent in 2005.

Frances Done, the chairman of the Youth Justice Board, said: "This survey shows that the majority of children are law-abiding and know right from wrong. Unfortunately, there is still a minority that can blight the lives of their community with their offending."

The Home Office minister Vernon Coaker said police had seized 3,400 knives in the past 10 months: "Only a small minority of young people are regularly involved in serious violence and we are determined to get knives and other weapons off our streets."