British Transport police have released video images of three primary school children building a massive barricade on a railway line.

The three youths put a shopping trolley, tyres, tree trunks, traffic cones and concrete tiles on the track, with a passenger train just minutes away.

The boys - aged six, seven and nine - were filmed by a member of the public who had contacted British Transport Police about the incident on the busy Liverpool to Manchester line.

Officers detained the three boys - they cannot be prosecuted because they are below the age of criminal responsibility.

Passenger trains pass by every 15 minutes

The incident occurred between Lea Green and St Helens Junction stations on 27 June.

Four trains an hour pass over that stretch of railway at speeds of up to 80mph.

Chief Inspector Peter Holden, from British Transport police, said: "It's extremely shocking.

"I shudder to think of the consequences of youngsters playing in such a dangerous environment.

"And then there's the consequences of a train hitting the barricade.

"A train would have come off the track and potentially caused fatalities and injuries."

Officers managed to dismantle the barricade before a train collided with it.

A police spokesman said: "Liaison has been maintained with the relevant youth offending teams, the children's parents and their schools.

"We would urge parents to be aware of what their children are getting up to in the school holidays as incidents like this pose danger to rail users and the offenders themselves."