Wildlife experts are on alert after a rising number of North American mink, one of the biggest threats to native British animals, were spotted on the River Thames.

The animals were first brought to Britain for fur farming but many escaped into the wild, where they have no natural predators and are aggressive hunters.

Because of the damage mink do to wildlife, fisheries and both wild and domestic birds, Natural England's advice is that they must be controlled by being trapped and then put down.

Retired mechanical engineer Peter Darch spotted a family of mink while he was repairing a barge near Abingdon Lock and reported the encounter to the Environment Agency.

He said: "As I was casting off, I saw a little flash and I took a closer look and there was this little nose looking at me. I opened up the barge hatch and there were at least half-a-dozen black mink inside."

Mr Darch, of Banbury Road, Oxford, added: "They're one of the most vicious predators that we have got - they kill just for the sake of killing."

Environment Agency spokesman Emma Sears said it had received several reports from people concerned that wildlife, including water voles, had been attacked by mink.

She said: "The agency supports the Berks, Bucks and Oxon Water Vole Recovery Project, based with the Berks, Bucks and Oxon Wildlife Trust.

"The priority for mink control are those areas which are deemed to have good water vole populations. The project relies on volunteer effort for mink control, mainly by keepers and landowners who can undertake control in the required way.

"We see the need to control mink as an unfortunate consequence of their introduction by man and they should be dealt with as humanely as any other mammal and not demonised for doing what comes naturally."

The vole project's conservation team leader, Graham Scholey, said mink had been a problem in Oxfordshire for the past 30 to 40 years.

He said: "There's still a way to go, because mink are widespread. It's a big challenge to try to reduce their numbers substantially."