Bodies found in a mass grave in Repton are thought to be Viking war dead after a team of archaeologists used a unique dating technique.

They have been working at the burial site in the village and believe the bodes are Viking Great Army war dead.

The University of Bristol's Department of Anthropology and Archaeology, which is led by Cat Jarman, carried out scientific research at the mass grave at St Wystan's Church, which contained more than 250 skeletons.

Although the remains were initially thought to be associated with the Vikings, radiocarbon dates had seemed to suggest the grave consisted of bones collected over several centuries. But the team has now found that this was not the case and that the bones are all consistent with a date in the late 9th century.

Excavations led by archaeologists Martin Biddle and Birthe Kjølbye-Biddle at the church in the 1970s and 1980s discovered several Viking graves and a charnel deposit of nearly 300 people underneath a shallow mound in the vicarage garden.

They found an Anglo-Saxon building, possibly a royal mausoleum, was cut down and partially ruined, before being turned into a burial chamber on the site.

Pictured is an overview of the charnel burial from the original excavations

One room was packed with the commingled remains of at least 264 people, around 20 per cent of which were women.

Among the bones were Viking weapons and artefacts, including an axe, several knives, and five silver pennies dating to the period 872 to 875AD.

While 80 per cent of the remains were men, mostly aged 18 to 45, with several showing signs of violent injury.

During the excavations, everything pointed to the burial's association with the Viking Great Army, but confusingly, initial radiocarbon dates suggested otherwise.

It seemed to contain a mix of bones of different ages, meaning that they could not all have been from the Viking Age.

Bones from the Repton charnel during excavations

Now, new dating techniques proves the remains are all consistent with a single date in the 9th century and therefore with the Viking Great Army as historical records state that the Viking Great Army wintered in Repton, Derbyshire, in 873AD and drove the Mercian king into exile.

Radiocarbon dating works by comparing the three different isotopes of carbon in bones. Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.

This means that although they are very similar chemically, they have different masses. The process means archaeologists can date bones more acurately.

It has also been used to date the extinction of the woolly mammoth and contributed to the debate over whether modern humans and Neanderthals met.

But is not just used in dating. Using the same techniques, we can examine ocean circulation and trace the movement of drugs around the body.

Mrs Jarman said: "The previous radiocarbon dates from this site were all affected by something called marine reservoir effects, which is what made them seem too old.

The life and times of the Tutbury Jinnie

"When we eat fish or other marine foods, we incorporate carbon into our bones that is much older than in terrestrial foods. This confuses radiocarbon dates from archaeological bone material and we need to correct for it by estimating how much seafood each individual ate."

A double grave from the site – one of the only Viking weapon graves found in the country - was also dated, yielding a date range of 873 to 886AD.

The grave contained two men, the older of whom was buried with a Thor's hammer pendant, a Viking sword, and several other artefacts.

He had received numerous fatal injuries around the time of death, including a large cut to his left femur.

Intriguingly, a boar's tusk had been placed between his legs, and it has been suggested that the injury may have severed his private parts, and that the tusk was there to replace what he had lost in preparation for the after-world.

This is one of the female skulls from the Repton charnel

Outside the charnel mound, another extraordinary grave can now be shown to be likely to relate to the Vikings in Repton as well.

Four juveniles, aged between eight and 18, were buried together in a single grave with a sheep jaw at their feet.

Next to them large stones may have held a marker, and the grave was placed near the entrance to the mass grave. At least two of the juveniles have signs of traumatic injury.

The excavators suggested this may have been a ritual grave, paralleling accounts of sacrificial killings to accompany Viking dead from historical accounts elsewhere in the Viking world. The new radiocarbon dates can now place this burial into the time period of 872 to 885AD.

Mrs Jarman, added: "The date of the Repton charnel bones is important because we know very little about the first Viking raiders that went on to become part of considerable Scandinavian settlement of England.

"Although these new radiocarbon dates don't prove that these were Viking army members it now seems very likely. It also shows how new techniques can be used to reassess and finally solve centuries old mysteries."