According to The Local, the magazine uncovered shocking scenes, including a dead piglet left lying on a walkway and a sow covered in wounds from the bars of a narrow cage in which she could not lie down.

New European Union rules, which have been in the pipeline for eleven years, finally came into force on 1 January this year after a six year grace period.

Among other things, the rules stipulate that pregnant sows can no longer be kept in a narrow barred cage or for the entire three month duration of their pregnancy, but must be transferred to a larger group pen after five weeks, before returning to an individual cage to give birth.

Individual cages must now by law be big enough for animals to lie down on their sides with their legs stretched out and must be fitted with toys to prevent the animals biting each others tails out of boredom.

Both these rules are consistently broken, activists told the magazine's reporter, who personally accompanied them on a night expedition to a pig farm in Vechta, near Bremen in north western Germany.

There, activists filmed sows kept alone and covered in wounds from forcing their legs through bars of cages which were too small for them and contained nothing to occupy the animals.

They photographed dead piglets lying on the walkway floors or stacked up in wheelbarrows ready to be carted off, wrote the magazine.

In their fight to raise awareness, the activists regularly pass on the photos and video footage they collect to the Animal Rights Watch (Ariwa) organisation where it is published on their website.

The European Union could now take Germany to court for breaking the rules, said Der Spiegel.

Even the ZDS central association of German pig producers admits that an estimated one third Germany's 15,000 pig farms have not implemented the changes - despite the six year grace period.

One reason for farms failing to comply with the rules is a lack of money, the German Agricultural Ministry told the magazine.

German pig farmers are under growing pressure to keep production costs as low as possible it said, as the European pork industry tries to sell its excess products on the world market.