A mother who helped police catch two teenage thugs after they snatched her son's iPod has condemned the criminal justice system for failing to prosecute them.

Teacher Amanda Jones chased the young thieves through the streets of Birmingham until they were caught and arrested.

But her efforts came to nothing when the authorities told her the offenders would not be prosecuted.

Mrs Jones, a teacher from Yardley, said: "I was shocked. The police did a fantastic job and caught these two thieves, but then they were just let off."

But David Blundell, Chief Crown Prosecutor, defended the decision to give the youths a final warning, the equivalent of a caution in cases involving juveniles.

He said: "It is more than just a talking to. It is more structured than that, and designed to stop them offending again."

The drama began as her 15-year-old son was walking down the Coventry Road, near the Swan Shopping Centre in Yardley. He was approached by two teenagers, one aged 14 and one aged 16, who told him to hand over his iPod, and jumped on to a number 69 bus. The victim then phoned his mother to tell her what had happened.

She reported the crime to the police, before getting into her car and picking up her son.

Together, they followed the route the bus took, and caught it up at a bus stop.

Mrs Jones told the driver what had happened, and asked him to stop until the police arrived. Then she headed upstairs to the top of the vehicle to confront the thieves.

They ran downstairs and attempted to flee on foot, with Mrs Jones in pursuit. But by this time, police in three cars were out looking for them.

The two young men were arrested and police found the iPod hidden in one of the offender's socks. But Mrs Jones was astonished the next day when she was told by police that there would be no prosecution.

Because there had been no explicit threat of violence, the case was to be classed as theft rather than robbery. As the two culprits also had no previous convictions, the pair would be issued with a final warning.

Mrs Jones said: "I couldn't believe it. We are always being asked to support the police, but it seems to be a wasted effort."

Yardley MP John Hemming (Lib Dem) added: "When thieves are caught and identified, the Government has established rules that result in the criminals being let off with a caution.

"Blair as usual is going for the wrong target . . . we as a society have to say that we will not tolerate these sorts of offences."