LONDON - Archaeologists have discovered a mysterious coffin-within-a-coffin from the same Leicester car park where England's King Richard III was famously buried for 500 years.

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 full intact stone casket with a lead coffin inside.

A stone coffin is pictured after its lid was removed at the Greyfriars dig site in this handout photo by the University of Leicester released July 29, 2013. [Photo/Agencies]

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," Mathew Morris, the site director of the University of Leicester Archaeological Services, was quoted as saying in the announcement of the find on the university's website.

"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 cm wide at one end and 30 cm wide at the other, the university said.

Laboratory tests will be carried out on the lead coffin, which was found inside the stone casket, before it can be opened but archaeologists suspect it may be one of the friary's founders or a medieval monk.

The grave could belong to one of three prestigious figures known to be buried at the friary, Peter Swynsfeld, who died in 1272; William of Nottingham, who died in 1330 - who both headed of the Grey Friars order in England - or Sir William de Moton of Peckleton, who died between 1356 and 1362.