Police have been urged to step up their efforts to tackle a bizarre sex craze sweeping North-East beauty spots.

Countryside workers say that dogging - where couples get their kicks out of inviting strangers to watch and sometimes join in with them having sex - is increasing in popularity.

There have been fears that dogging is blighting some areas and is linked to crimes such as blackmail, violence and drug taking.

An investigation by The Northern Echo has found scores of locations in the region popular with "doggers", many in beauty spots and country parks.

Within seconds of logging onto the Internet reporters were able to find 11 alone in County Durham, six in Cleveland and nine in the Sunderland area.

One website posting referring to the Hardwick Hall country park, near Sedgefield, County Durham, says that its secluded car parks are a favourite spot.

It says: "One for mainly couples and single guy watchers.

"Old bill cruises through from time to time but not a problem. Wait until dark though as families use the park in the daytime."

Another lists a car park in the picturesque village of Newton-under-Roseberry, near Great Ayton, as being popular with couples and also describes "a toilet with peepholes".

Gamekeeper Brian Pavey, chairman of crime prevention group Darlington rural watch, said his members had complained to police about a number of locations in which such activity was taking place.

He said: "This thing is growing in frequency and unless it is stopped by the police it will definitely continue spreading.

"The people involve advertise themselves and arrange to meet in cars in secluded places and their numbers are increasing."

Doggers can be breaking the law if their actions are deemed by police to be indecent or cause harassment, alarm or distress to members of the public.

But of greater concern is thought to be the safety of those actually taking part.

Dr Richard Byrne, a senior lecturer in countryside management at Harper Adams College, in Shropshire, who has carried out research into the issue, said many crimes linked to dogging were not being detected either because of a lack of resources or because they were simply not being reported.

He said: "The police are so overstretched that by the time they get to a specific place the people involved have moved on.

"Doggers are a mixture of exhibitionists and voyeurs but some of those involved don't know what they are getting into as there is a lot of drink and other things flying about.

"There is a risk in terms of them being blackmailed into acting against their will or possibly attacked, but this is never reported to the police anyway."

The phrase dogging originated from explanations often given to police from those who are watching on who say they are "just walking the dog".

Many meetings for sex are filmed and appear on Internet websites, The Echo has learnt.

Inspector Neil Malkin, of County Durham police, said police were working with their partners to try and manage the problem and ensure that such behaviour did not cause concern to the public.

He said: "We do have a number of public sex areas across the region but we are not unusual in that.

"We are also aware of these websites and have advertised on them to warn that we are patrolling these areas and will enforce the law if necessary.

"There are anti-social behaviour issues here and some of the locations in use have issues of drug misuse and are open to drug dealing."

Insp Malkin added: "People can talk to us in confidence if they have been harassed or attacked or subjected to any kind of criminal behaviour and we would ask them to come forward if this is the case."