Six killers who were hanged and buried in the grounds of Cardiff prison are to be exhumed to make way for a new cell block.
Cardiff jail holds three-quarters more prisoners than it should
The Victorian prison was recently named as the sixth most overcrowded in England and Wales, and a new cell block is planned on the site of the graves to ease the problems.
Prison officials are trying to trace relatives of the dead men so their remains can be reburied in public cemeteries.
John Gorbelt, George Roberts, Howard Grossley, Evan Evans, Ajit Singh and Clifford Wils were all executed for their crimes with Wils the most recent in 1948.
Grossley was a Canadian soldier convicted of murdering a woman in Porthcawl, south Wales, and hanged in 1945.
No date has yet been fixed for the exhumation - which will be granted by the Home Office - but the bodies will be removed at dawn.
A prison service spokesman said: "This procedure is not common but it does happen from time to time - as prison populations rise more space is needed to accommodate prisoners.
"We want the relatives of those prisoners to be able to have them moved wherever they want. This is something that is always dealt with sensitively."
If no family members come forward the remains will be re-buried at another site within the prison grounds.
Cardiff jail - which has a capacity of 722 - was not the only Welsh prison mentioned for overcrowding in the Prison Reform Trust report.
Swansea - which holds 346 people - was just ahead, being the fifth most overcrowded.
Both had three-quarters more prisoners than they should have.
The Trust said the cramped conditions meant many prisons failed to meet the recommended average of 24 hours a week of purposeful activity for each prisoner.
But the Trust said the situation in Welsh prisons was still better than the majority of those in England.
Relatives of the six who will be exhumed at Cardiff jail have until Friday, 31 October, to contact the authorities.
They have been asked to write to WG Loader, Chief Estate Officer, HM Prison Service, Abell
House, John Islip Street, London SW1P 4LH.