Animal rights activists are using drones to spy on farms – causing distress and possible harm to livestock, farmers say.

The National Farmers Union (NFU) says campaigners are flying remote-control aerial machines above its members’ property for surveillance.

The activists are breaking the law if the drones come within 150ft of a person or a building.

Pigs will charge and run as a herd when scared, destroying electric fences and escaping on to roads

Farmers believe it may lead to serious injuries for staff, animals or the public.

There has been a particular problem at pig farms and abattoirs where protesters have demonstrated and tried to disrupt business.

Drones have also been used to observe chicken and dairy farms, and find badger traps to sabotage during the cull.

Farmers worry animals may be alarmed by the sight and sound of a drone. Pigs will charge and run as a herd when scared, destroying electric fences and escaping on to roads. Pregnant sows can abort piglets through the shock.

Chickens can panic, mistaking a drone for a bird of prey. They will flock together, and if stressed and pushed they can suffocate.

The NFU’s Rupert Weaver said: ‘Farmers are reporting a lot of problems with drones. Animals are unpredictable and you just don’t know how they are going to react. There are some who are deliberately using drones maliciously while others have not thought of the dangers.’

The National Farmers Union (NFU) says campaigners are flying remote-control aerial machines above its members’ property for surveillance

Des Allen, a pig farmer from Coddington, Nottinghamshire, told the NFU council last week that activists were ‘the bane of our life’. He said: ‘My guys are confident they have had drones looking at us. [Drones] are a serious problem and they are invading our privacy.’

Albert Carter, of Alton, Hampshire, said his sheep had been disturbed by a drone. ‘I didn’t see anyone – that is the trouble. The sheep were spinning in circles. This was 100 per cent deliberate.’ Police were told but no one was arrested.

Farming leaders are calling for further restrictions on drones.