Archaeologists have discovered a mysterious coffin-within-a-coffin under the Leicester car park where the skeleton of England's King Richard III was famously uncovered more than 500 years after he fell in battle.

Experts returned to the site of the former Grey Friars Church at the beginning of July to learn more about the area, only to find a fully intact stone casket with a lead coffin inside.

The same team of university archaeologists had previously discovered English King Richard III's skeleton in the remains of the friary church, now located under Leicester City Council's social services department.

"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," said Mathew Morris, the site director of the University of Leicester Archaeological Services.

"None of us in the team have ever seen a lead coffin within a stone coffin before.

"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."

It took eight people to lift the stone lid from the outer coffin, which is more than 2 metres long, 60 centimetres wide at one end and 30 cm wide at the other, the university said.

While the lead coffin is in fairly good condition, the narrow end is somewhat degraded and the occupant's feet are visible.

"Apart from that there are no identifying features. It is like an elaborate game of pass the parcel!" the university said on its website.

Laboratory tests will be carried out on the lead coffin before it can be opened.

The grave is thought to be a "high status burial" and could belong to a medieval knight, Sir William de Moton of Peckleton, who died between 1356 and 1362.

Alternatively, it could belong to one of the friary’s founders - Peter Swynsfeld, who died in 1272, or William of Nottingham, who died in 1330.

ABC/Reuters