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Archaeologists have uncovered a mysterious coffin-within-a-coffin while excavating the final resting place of King Richard III.

The University of Leicester team opened the lid of a medieval stone coffin this week during the final week of their second dig at the Grey Friars site where the British king was found last September.

The stone coffin is thought to contain one of the friary's founders or a medieval monk.

Once opened, archaeologists were surprised to discover a second lead coffin inside of the stone coffin.

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"None of us in the team have ever seen a lead coffin within a stone coffin before," Grey Friars site director Mathew Morris of the University of Leicester said. "We will now need to work out how to open it safely, as we don't want to damage the contents when we are opening the lid."

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The archaeologists suspect that the coffin belongs to one of the three prestigious figures known to be buried at the friary.

"The coffin could contain William de Moton, Peter Swynsfeld or William of Nottingham – who are all important people," Morris said. "Swynsfeld and Nottingham were heads of the Grey Friars order in England."

Sir William de Moton of Peckleton was a prominent 14th century knight.

However, there are many other nameless people who were also buried at the church and the identity of the person in the lead coffin may never be revealed.

"The stone coffin was always the big thing we wanted to investigate during this dig," said Morris. "For me, it was as exciting as finding Richard III. We still don't know who is inside – so there is still a question mark over it."