Scientists have discovered the remains of three humans – including a child-of-three – that appear to have been killed for food, their bodies butchered and then eaten.
The bones showed precision cuts to extract the maximum amount of meat and the skulls had been carved into cups and bowls for drinking and eating.
The fragments – which are 14,700 years old- are thought to be the oldest examples in the world of skull cups and the first evidence of ritual killing in Britain.
What is particularly horrific is that at the time, humans knew how to bury their dead and so were not savages meaning the remains are most likely the result of premeditated cannibalism.
"At the time life was very tough," said Professor Chris Stringer, a palaeontologist at the Natural History Museum, who helped excavate the skull cups.
"Cannibalism would have been a good way of removing groups competing with you and getting food for yourself.
"There was also a feeling that if you ate your enemy you gained some of his power."
At the time Britain was just emerging from the Ice Age and the cavemen – believed to be Cro Magnons originally from France – would have come to Britain from the Netherlands in summer, probably following animal herds migrating across land that is now the North Sea.
Just a few hundred strong, the hunter gatherers would have mainly lived off reindeer and horses but when times got tough it is believed they would have fought and eaten competing groups.
Prof Stringer said they were not savages and knew exactly what they were doing.
"What is more sinister is that these were quite sophisticated hunter gatherers – very like us," said Prof Stringer.
"They could make tools and painted cave art. They also had quite complex burials for the people they were not eating, treating the dead with reverence.
No one can be sure, exactly what happened but the three cadavers, found in Gough's Cave, Somerset, show signs of being chopped up for food.
The 41 pieces of bone, found in 1987 but only just fully analysed, were broken up to remove the marrow and cut marks shows that they were meticulously cleaned to remove all the soft tissues.
Most gruesomely the skulls were shown to have been smashed into cups, the hair scalped from the head, and the tongues and eyes gouged out.
The sharp edges were also cleaned up so that they could be used a drinking cup perhaps to consume "blood, wine or food" during the meal.
But once the feast had been finished the scientists believe that the cannibals discarded the bones and the skull cups and moved on.
While the tradition of using brain cases as drinking cups or containers has been documented in a variety of past, and even present, communities, archaeological evidence is extremely rare.
The use of skulls as cups has been known to a be feature of human history for centuries.
Some Buddhists and Hindus today still use them in rituals.
The three skull-cups identified among human bones from Gough’s Cave, Somerset, are the only physical evidence of skull-cups from our forebears in the UK.
Dr Silvia Bello, palaeontologist and lead author, described the production of the skull-cups.
"We suspected that these early humans were highly skilled at manipulating human bodies once they died, and our research reveals just what great anatomists they were.
"The cut-marks and dents show how the heads were scrupulously cleaned of any soft tissues shortly after death.
"The skulls were then modified by removing the bones of the face and the base of the skull. Finally, these cranial vaults were meticulously shaped into cups by retouching the broken edges possibly to make them more regular. All in all it was a very painstaking process given the tools available".
A replica of one of the specimens will go on display at the Museum in London on 1 March 2011 for three months.
The research is published in the journal Public Library of Science One.