A Strathclyde Police spokesman said the child was taken to Baird Street police station from Barlinnie Prison in Glasgow and found to be in possession of a quantity of controlled drugs.

The drugs are said to have a street value in the region of £150 to £175. Inside Barlinnie they would be worth about £500.

Campaigners say that children are often used as drug couriers

The boy, who was accompanying a woman on a visit to an inmate, had not got as far as the visiting hall when they were stopped.

A report will be submitted to the Reporter to the Children's Panel.

Glasgow councillor Gaille McCann - a founder member of Mothers Against Drugs which was set up last year after the death of 13-year-old Alan Harper - said a stricter regime was needed in prisons.

"Kids are being used as couriers. They (inmates) will stop at nothing to get their drugs in - it is absolutely immoral," he said.

Separate seizure

Barlinnie, on the eastern outskirts of Glasgow, is Scotland's biggest jail.

The forbidding Victorian complex houses 1,100 inmates - a mixture of men on remand and those from the Glasgow area serving four years or less.

Although classed as a top-security prison it is not used for this purpose.

Visiting times are no longer restricted to one particular time of day, and in many cases families book an appointment for a specific time beforehand.

The Scottish Prison Service declined to comment on the incident.

Earlier on Thursday, police said a 15-year-old girl had also been detained in connection with a late-night seizure of heroin worth nearly £140,000 at nearby Motherwell railway station.