Ravens used to be a common sight in Britain

The move has been prompted by the "suspicious" disappearance of three rare chicks from the Derwent Valley.

Now an array of measures including unmarked police cars and the force helicopter are being made available to protect the birds.

The Nestwatch scheme, set up by groups including the Royal Society for the Protection of Birds, the National Trust and police in South Yorkshire and Derbyshire, was first established to protect the nests of peregrine falcon and goshawks in the Derwent valley.

But the loss of the raven chicks has put members on heightened alert for all rare birds.

This is an exceptionally upsetting event, it is a line in the sand Mark Thomas, Peak Nestwatch Scheme

Mark Thomas, from Nestwatch, said: "These were three healthy chicks. They were not old enough to leave the nest so their disappearance is very suspicious."

Until recently ravens were confined to the north west of England but have begun to nest further south.

There are only two breeding pairs known in the Derwent Valley.

Mr Thomas said: "These birds do no harm to anyone. They are huge, majestic birds, very spectacular in flight.

"We have been told unmarked police cars will be in the area in future weeks and have been offered the police helicopter if something happens in a remote area.

"We will be using night-vision goggles to monitor nests 24 hours a day, along with CCTV and special marking dye known as Smart Water.

"Anybody foolish enough to interfere with any nests will do so at their own risk."