Conservationists at England's only wild beaver colony denied a beaver which has set up home on the River Thames near Oxford is a fugitive from their estate.
The new beaver, which has been living for more than three months at a secret location, has been identified by bite-marks in trees.
Experts have said the mysterious arrival is from a new source of European beavers, and is not one of six introduced three years ago to the Lower Mill Estate - a luxury waterside housing development, in Gloucestershire.
Tony, Cherie, John, Pauline, Gordon and Sarah were the first of their species to be seen in Britain since being hunted to extinction in the 16th century. They have since gone on to breed.
Although a 3ft-high fence was erected around the site to keep the beavers inside, two escaped, but were quickly recaptured.
The Environment Agency said the location of the beaver's lodge was being kept secret to avoid it being disturbed or harmed.
The estate has insisted the beaver is not one of theirs, but has offered its help to catch it.
Dr Simon Pickering, Cotswold Water Park director of conservation, said: "It's not from Lower Mill Estate as far as we're concerned.
"It has probably come from a zoo or a small private collection, but it's difficult to say.
"When this one turned up the Environment Agency contacted us. We went to look and confirmed there was a beaver because of the classic signs, mainly trees recently being gnawed."
He added: "They are very good engineers and wonderful builders, but the Thames was just a bit too big for one to build a dam."
The European beaver is not as keen on dam-building as the Canadian species.
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