Cumbria Police have revealed they targeted three bars in the city as part of a campaign to crack down on underage drinking.
A 16-year-old was sent into all three premises after police received intelligence they were selling alcohol to underage young people.
Of the three bars visited, one of them served the teenager without checking their age.
The bar which failed the test was issued with a £90 fixed penalty notice.
“I am disappointed that licensed premises continue to serve alcohol to underage drinkers, despite the fact that it puts the health of children and young people at risk.
"However, I was pleased to note that two out of the three bars tested did the right thing and challenged the young person and requested identification. We need all licensed premises to adhere to these standards.
“Our advice is to always ‘Ask, Challenge, Check’ – if in doubt, do not serve.”
– Sergeant Richard Higgin
Anyone with any information of premises who sell to underage children are asked to contact Cumbria Police on 101, or contact Crimestoppers anonymously on 0800 555 111.
The Press Association have approached councils across the UK and found children as young as four being referred by education and children's services to alcohol and drug specialists.
In the Freedom of Information request, more than half of under-13s - 59% - received treatment for cannabis misuse, while a third were treated for alcohol misuse.
A small number abused solvents.
Eight-year-olds had been referred to services in Waltham Forest and East Ayrshire, while nine-year-olds had been referred in Herefordshire, Liverpool, Oxfordshire, Rutland, the Scottish Borders and West Berkshire.
People in Cumbria are being urged to call the police if they see anyone they've seen drinking attempting to drive home over the festive period.
Police say they will treat any tip offs as a priority and say the focus of this year's annual drink driving campaign is the responsibility of ordinary people to help keep the roads safe this Christmas.