The 15-year-old was fast asleep high up on the crane's arm

Police and firefighters were called to a building site in south east London, after a passer-by spotted the girl.

The unnamed 15-year-old had apparently left her home near the site, climbed the crane and walked across a narrow beam while remaining fast asleep.

The girl was rescued unharmed in the incident which happened on 25 June but has only just been revealed by police.

Mobile phone

Police were initially called at 0130 BST amid fears she was about to throw herself off but when a firefighter scaled the crane he found her curled up asleep on top of a concrete counterweight high above the ground.

Fearing to wake her in case she should panic and fall off the arm, the firefighter is understood to have found her mobile phone and called her parents from the top of the crane.

They then phoned her to wake her up.

She was eventually brought down by hydraulic lift and taken to hospital for checks but was found to have suffered no ill effects.

Expert Irshaad Ebrahim, of the London Sleep Centre, told the Times newspaper he had treated people who had driven cars and ridden horses while asleep.

He said one patient had even attempted to fly a helicopter.

Sleepwalking affects one in 10 people at least once in their lives.

Most incidents are short and are not dangerous but it can sometimes result in injury.