A new DNA test can restore at least part of the identity of long-dead people who left no trace of their image, scientists reported on Monday.

The technique has revealed the hair and eye colours of unknown individuals slaughtered as sub-humans by the Nazis and of a mystery woman buried alongside monks in a mediaeval crypt, they said.

"This system can be used to solve historical controversies where colour photographs or other records are missing," said Wojciech Branicki from Poland's Institute of Forensic Research in Krakow.

The system, called HIrisPlex, is a fine-tuned version of a tool presented two years ago that looks at tiny variations in the DNA code, and converts these into probabilities for hair and eye colour.

It can be used on teeth and bones, whose DNA survives better than soft tissues, and which explains the interest in using it for ancient forensics.

Reporting in the journal Investigative Genetics, the researchers first tested it on a tooth taken from the remains of General Wladyslaw Sikorski, who led Poland's government-in-exile in Britain in World War II before dying in a plane crash in 1943.

Sikorski's body was disinterred from a cemetery in Newark, England, in 1993 for reburial in pomp in Krakow, but was exhumed once more in 2008 for further examination to sound out a theory that he had been poisoned, shot or strangled.

Analysis of the genetic code from the tooth gave a 99-percent likelihood that Sikorski had blue eyes, and an 85-percent likelihood that he had blond hair.

Both tallied with contemporary descriptions of Sikorski and with paintings of him made many years after his death (no colour photographs of him are known to exist).

HIrisPlex also gave a partial fix on the identity of 12 people who had been killed in a prison in Nazi-occupied Poland in 1942 and whose names have never been known.

Their skulls were sent to wartime Vienna, where they were exhibited in the city's Natural History Museum as examples of "sub-humans."

DNA from the skulls showed a very high probability that the individuals had blue or brown eyes, and hair that ranged from light brown to blond, a finding consistent with a Polish European ancestry.

A third test helped shed light on a find at a Benedictine abbey in Tyniec, near Krakow, where archaeologists were stunned to find the skeletons of two women among 17 skeletons that had been expected to be of male abbots.

Tests on one of the samples found the woman had brown eyes or dark blond or brown hair.