Historians believe human bones which were unearthed by landscape gardeners at a house in Purley will be found to date back to Anglo-Saxon times.
Police were called to the home in Riddlesdown Road last Monday after workers discovered a human skull and a thigh bone while building a driveway.
Officers are investigating how the bones ended up at the house with historians saying it is likely to be part of a Saxon burial ground.
Local history experts have said the the fact only the head and another bone was found could be because the skull belonged to a beheaded Saxon criminal, but this is thought to be unlikely.
Croydon Natural History and Scientific Society’s archaeology secretary Jim Davison said: "That whole area has been used as a burial area.
"You could have a decapitated body with the head by the feet or something but to determine that you would need to examine the bones in the neck."
David Brooks, museum assistant at Bourne Hall Museum in Epsom, said: "It’s possible it is a beheaded criminal but normally the head is kept with the body.
"I think if it is Saxon then it is likely to be a disturbed burial that has been moved around.
"If it was farmland before it was built on then it has been ploughed over and this could have moved the bones about and smashed them up."
Police enquiries centre around trying to establish the age of the bones.
Investigating officer Detective Sergeant Kenny Windsor, from Croydon police, said: "Any requirement for an investigation will be determined by the age of the bones and we're seeking the assistance of anthropologists to try and establish this, but we may also have to use carbon dating techniques to confirm their age.
"We are carrying out other enquiries in the meantime."
Wallington landscape gardener Terry Jobson said his workers were using their digger at the property last Tuesday morning and had just dug into the soil when they saw the skull.
Mr Jobson, of B&T Jobson Landscape Gardeners, said: "If it is 70 years old or older then it is not a murder enquiry and all they are going to be able to tell us is whether they are less than 70 years old or ancient."
The Carpenter family have lived in the house for two years and were out when the discovery was made.
They got a call from one of the workmen telling them what had happened.
Mother-of-four Alison Carpenter, 41, said: "The builders got extremely freaked out by it.
"I thought it is a good job we have not been living here long otherwise we might have been arrested.
"It would be nice to know who it is and what happened to them."
Dad Michael Carpenter, 46, said: "The kids thought it was brilliant.
"It has certainly livened up the Easter holidays."