Gathering dust in police files is a dossier containing the fingerprints of the most unlikely criminal gang - half a dozen chimpanzees and a pair of orang-utans.

Their dabs were taken during police raids at the Ape House at London Zoo and at Twycross Zoo in Leicestershire. The operation, by fingerprint experts from Hertfordshire police, took place in 1975 at a time when there was growing concern over unsolved crimes. It concluded that chimp dabs looked exactly the same as ours, but did not link them to any specific offence.

The chimp file is likely to be re-examined in the light of new evidence yesterday that criminal investigations in Australia may have been hampered by the presence of koala fingerprints at the scenes of crimes.

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Maciej Henneberg, a biological anthropologist and forensic scientist at the University of Adelaide, said that the marsupials had fingerprints which were so close to those of people that they could easily be mistaken by police.

While handling koalas in Urimbirra wildlife park, near Adelaide, Mr Henneberg noticed their fingers carried ridged patterns of loops, whorls and arches like those on a human hand.

"It appears that no one has bothered to study them in detail," he said. "Although it is extremely unlikely that koala prints would be found at the scene of a crime, police should at least be aware of the possibility."

The animal connection did not surprise Frank Wheeler, head keeper of small mammals at London Zoo, who clearly remembers the arrival of the police squad 21 years ago.

The chimps, all juveniles aged around six or seven, did not struggle as their digits were dusted and pressed on to sticky fingerprint tape. "They sat there quite happily," he said.

As brachiaters (animals which move sideways by swinging hand over hand), the orang-utans have tiny thumbs, which put them out of the frame.

Mr Wheeler disputed the Australian evidence that koala prints looked human. "Their hands have been adapted for climbing," he said. "Three digits face forwards and two face sideward."

The police operation in 1975 was led by Steve Haylock, now with the City of London police fingerprint bureau. He said the exercise was carried out because police officers habitually referred to spoiled fingerprints as "monkey prints".

The zoo expedition proved this was nonsense. Mr Haylock said: "If you passed a chimpanzee print to a fingerprint office and said it came from the scene of a crime they would not know it was not human."

Among those finger-printed was a face familiar to millions of television viewers; not as a wanted villain but as a star of PG Tips tea commercials. The police team briefly considered taking prints from gorillas but thought better of it.

There are no koalas in Britain. The last one was taken out of London Zoo several years ago and deported to Portugal. It had become lonely and was not under suspicion of a criminal offence.