When they were not dining on mammoth meat, Neanderthals were munching nuts, berries and roots, research suggests.

Around 20% of the ancient humans' diet was vegetarian, analysis of their bones reveals.

Scientists looked at a vast array of 40,000 to 45,000 year-old bones of animals and several Neanderthals from two excavation sites in Belgium.

By studying collagen in the bones, they were able to find clues about the creatures' eating habits.

Contrary to what had previously been thought, the Neanderthal diet differed markedly from that of predators such as cave hyenas and lions, the research showed.

Neanderthals chiefly hunted large plant-eating mammals such as mammoths and woolly rhinos - but plants made up about a fifth of what was on their menu.

Lead scientist Professor Herve Bocherens, from the University of Tubingen in Germany, said: "In this study, we were able for the first time to quantitatively determine the proportion of vegetarian food in the diet of the late Neanderthals."

The research, published in the Journal of Human Evolution and Quaternary International, suggests that the extinction of the Neanderthals around 40,000 years ago was not linked to their reliance on certain kinds of food.

"We are accumulating more and more evidence that diet was not a decisive factor in why the Neanderthals had to make room for modern humans," said Prof Bocherens.

Neanderthals were a human sub-species that lived alongside the ancestors of people living today in Eurasia for some 5,000 years.

Their inability to compete with early modern humans is widely believed to have led to their extinction.