Jack Bell, 19, of West Princes Street, made the threat after taking the phone from the boy who was waiting for a lift from parents outside a hall on East Princes Street on March 24 this year.

Bell’s conviction arrives days after a high-profile case in which a 38-year-old Greenock dad was jailed for six years after admitting killing a knife-wielding thief who stole his son’s iPhone.

The incidents have sparked warnings from members of the Helensburgh community, who highlighted the dangers faced by children carrying mobile phones in the street.

Councillor Vivien Dance, chairwoman of the Helensburgh and District Crime Prevention Panel, said: “It is particularly tragic when young people resort to crime over a mobile phone without realising the impact this will have on their lives for years to come and perhaps young people reading these recent stories will stop and consider the consequences of their actions for themselves, and their families, and how things can easily escalate to loss of life over a phone.” Inspector Claire Miller, of Helensburgh Police, added: “If you do find yourself facing a thief, don’t be brave, give up your property can be replaced.” At Dumbarton Sheriff court last week, Bell appeared from custody to be sentenced for the offence The procurator fiscal depute Scott Simpson told the court Bell — who was under the influence of drink or drugs — approached three 14-year-old boys as they stood outside a hall in East Princes Street waiting on their parents arriving to give them a lift home. He demanded one of the teenagers empty his pockets but the schoolboy refused.

Bell asked the teenager if he could use the mobile to call his mum, but was told there was no credit in the phone.

At that point Bell became aggressive and said: “Come on just let me use your phone” and the boy handed over the phone to him.

However, Bell realised the phone had a pin code lock on it and demanded that the youngster unlock it before threatening him, saying: “Hurry up or I’ll stab your neck.” Whilst the incident was ongoing, someone within the nearby hall had contacted the police.

Bell then asked another of the youths if they wanted to fight before backing down — apologising to them before shaking their hands and walking away.

One of the teenagers was able to identify Bell who, when questioned by the police about his involvement, denied making any threats to the teenagers. He added: “I only asked for a shot of the wee man’s phone.” Kirstin MacCrae, Bell’s solicitor, said in mitigation: “He did ask to use the phone but has no recollection of making any threats. He does accept that he must have done and caused considerable fear and distress. He now wishes to apologise.” The solicitor informed the court that Bell is currently serving a period of detention for another matter and his earliest release wasn’t until December this year.

Imposing the further custodial sentence, Sheriff John Hall said: “This is a very serious matter. Your behaviour on the afternoon in question must have been terrifying for the complainer and his friends who were simply waiting on a lift home.

“I’m satisfied that, because of the seriousness of these offences and your record, only a custodial sentence is appropriate.” Sheriff Hall sentenced Bell to 10 months in a young offenders institution.

When asked to comment on the risks faced by youngsters in Helensburgh when using mobile phones and other devices in public, Inspector Miller reiterated the importance of remaining vigilant.

She said: “Always walk with your head up, be alert and look out for dangers, that way, if you see something or someone that concerns you, you can keep your distance and walk away. If you are lucky enough to have an expensive mobile phone or similar gadget keep it discreet.

Cash should be kept out of sight. Generally we can go about our business and not have to experience a theft but it is best to be prepared. If you do find yourself facing a thief, don’t be brave, give up your property, items can be replaced.” A spokeswoman for Agryll and Bute Council said the safety of pupils at Hermitage Academy is paramount to staff both in and out of school.

She added: “Staff at Hermitage Academy are keen to promote the personal safety message to pupils, through personal and social education and in class. The school also works with community police officers who come into school for talks about how students can stay safe.” “Our policy on mobile phone use in schools is that they should be switched off once pupils are within the school building, and should only be used outside the school at break or lunchtimes. The only exception is in the case of an emergency and then with the approval of a member of staff.”