Police Scotland has written to secondary schools across the country warning parents and teachers that criminal gangs are targeting teenagers to act as “money mules”, as a senior officer warned youngsters are increasingly recruited online to move the proceeds of fraud through their own bank accounts.

Recent inquiries into “vishing” fraud in Scotland – where individuals are tricked into revealing financial information over the phone – have revealed that organised crime groups are targeting schoolchildren to work as mules, a form of money laundering that can carry a 14-year prison sentence but which young people often do not realise is illegal.

The warning follows the arrest and charge earlier this month of 29 people, including nine teenagers who police believe were working as mules. The UK fraud prevention service Cifas predicts figures released next month will confirm a further rise in under-21s involved in the activity.

DI Graeme Everest of Police Scotland’s organised crime and counter terrorism unit (OCCTU) warned parents that “across the country, young people are increasingly being asked by fraudsters to receive and send money through their own bank accounts”, with schoolchildren being targeted online but also vulnerable to approaches at youth clubs, sports centres and outside schools.

Everest said adverts on Facebook and other social networking sites, as well as group invitations on WhatsApp, promised “easy money” and “investment opportunities”, tricking young people into believing they are becoming involved in a legitimate financial occupation.

“There is usually one individual within the organised crime gang whose role it is to recruit: they do the adverts and make sure they have a number of people ready to be used at a specific time. At the same time as the fraudster is committing the crime, they’ll be in contact with the mule herder who will then be running the mules around various money service bureaus or outlets in order to get them to cash out the money or buy high-value items with it.”

As well as keeping a lookout for young people making unusually large transactions, Everest also warned parents to be alert to changes in purchasing behaviour.

“Mules will often be asked to buy watches, computers, telephones, iPads, anything that can be sold on for a specific value. The fraudsters tend to change the products they are using to commit the fraud regularly for fear of being traced, so if they then get a computer that’s been bought by a mule it’s untraceable back to them.”

Cifas data revealed a 26% increase in the number of under 21-year-olds implicated in money-mule activity in the first nine months of 2018. Simon Fell, the organisation’s director of external relations, said: “Social media is exacerbating the issue, with ‘get rich quick’ schemes, fraudulent WhatsApp groups and other techniques increasingly targeting younger demographics.

“A popular method of recruitment is inviting young people to join WhatsApp groups, and then asking them for access to their accounts or to provide their card details. If they decline, they are then asked to leave the group. In some cases gangs have threatened the mule and their family when they have tried to back out.”

Police Scotland has not approached any of the social networking sites involved directly, although UK Finance, the trade association for the financial services sector, said other forces in England and Wales had done so. Everest said: “It is a very difficult thing to regulate because as soon as one advert is taken down another will appear in its place.”

An anti-fraud system to help track suspicious payments and identify money-mule accounts, Mule Insights Tactical Solution (MITS), was recently launched by Pay.UK and Vocalink, in cooperation with UK Finance.