Youngsters risking their lives by lying down, or “planking”, on the walls of Thornton viaduct have been warned they are playing a “very dangerous game”.

Councillor Valerie Binney (Con, Thornton and Allerton ) said she was horrified when she was told girls and boys as young as 12 had been seen.

They scale the five-foot wall from the bridge deck to lie on it, with nothing to protect them from a 100ft drop.

Coun Binney said a resident told her she had seen teenagers playing on the walls on Thursday. Another resident told the Telegraph & Argus he regularly sees youngsters sitting on the walls, dangling their legs over the edge. And graffiti on the exterior wall of the viaduct appears to suggest the perpetrator leaned over to paint it.

Coun Binney, a grandmother of three, said: “Apparently it is called planking. They were lying on the walls.

“If they are doing that on the walls of Thornton viaduct it wouldn’t take much for them to slip and fall off and that would be the end of them. It is extremely high.”

She said she would urge any young people thinking of planking to “think again”.

She said: “It is a very, very dangerous game to play. A few children together can dare one another. I was horrified when I heard. They wouldn’t survive if they fell off there.

“The warning is not just to the children it is to the parents as well – they can drill it into them.”

She said she thought a sign at either end of the bridge warning of its height could act as a deterrent.

A resident of Alderscholes Close, next to the site, said: “The trouble is they are sitting on the wall with their legs dangling down. They scare the life out of us. It’s school holidays now so it can be more or less every day, especially evening time. Somebody has daubed it with paint, it’s ridiculous.”

Walkers at the viaduct yesterday said they did not want to see it closed due to risks taken by the young people.

The viaduct forms part of the Great Northern Trail, which is maintained by transport charity Sustrans.

A spokesman for Sustrans said: “Building large barriers or covering the Thornton viaduct would make it far less attractive and as such Sustrans has no plans to do this.”