A lead coffin unearthed from the same ruins of an English medieval church that yielded the remains of England's King Richard III has been opened and presented a surprise, archaeologists say.

Instead of a knight or a church official -- the double coffin of a lead casket inside a larger stone one suggested someone of importance -- the coffin was found to contain the skeleton of an elderly woman.

And very likely a woman of some importance, researchers say.

"A grave like this -- very elaborate stone sarcophagus, lead inner casket buried in a very prominent position in the church, close to the high altar -- you've got to think this person was important," says excavation leader Mathew Morris, a University of Leicester archaeologist.

The grave was found underneath a present day parking lot on the grounds where the medieval church, Grey Friars Friary, once stood.

In 2012, excavations uncovered a man's skeleton, hastily buried in a rough grave, which was subsequently identified as that of King Richard III, who was killed in 1485 during the Battle of Bosworth Field.

The mysterious double coffin was found very close to Richard's grave in 2013.

The identity of the elderly woman in the coffin remains a mystery, and church records from the time have yielded no clues, the researchers say.

Whoever she was, she will probably "forever remain anonymous," says Morris.

Still, the researchers say, some things can be assumed about her lifestyle.

Analysis of the bones showed she ate a protein-rich, highly varied diet that included large amounts of fish.

Such a diet suggests she was a person of wealth, able to afford expensive foods including fish, meat and game.

The fact she was wealthy and that she was buried not long after the church was completed in 1250 -- indicated by radiocarbon dating of the remains -- suggest she may have been a high-status church benefactor, the researchers say.

She was not the only woman buried in the church grounds, they add; in fact, Richard is the only male among the four burials examined at the site so far.

It is not unusual to find women buried in areas of church grounds normally reserved for noteworthy persons, or even within the church itself, Morris suggests.

In excavations at another medieval church in Leicester, "the burial of men and women inside the church was broadly equal," he says.