The 12 kits are flourishing at the 550-acre Lower Mill Estate near Cirencester, Gloucestershire – a sustainable luxury home complex designed by 30 of the world's top architects.

Experts believed six beavers introduced there three years ago had bred in January, after noticing tell-tale signs. But no one had seen the kits until now.

Estate owner Jeremy Paxton brought three pairs – called Tony and Cherie, Gordon and Sarah, John and Pauline – to Flagham Fen Lake in 2005.

Beavers were hunted to extinction in the 16th century because their throat glands were thought to be medicinal.

Estate spokesman Alistair Bankier said: "We are very excited – these are European beavers and not as aggressive as their American counterparts. People who live around the estate began to see the kits so we asked a professional wildlife photographer to camp out to get proof.

"It is difficult to spot them – you need to be there between 4am and 6am to capture images of the beavers. If they see humans that's it – they go back underwater.

"We've managed to get 20 minutes of footage. It shows the kits frolicking together with the mother beaver, swimming and dragging wood.

"Everyone is very proud that these kits have been born as conservation is part of the identity of the estate. There are over 6,000 protected wildlife species at Lower Mill and the beavers are just part of it."

With price tags up to £10m, stars such as Brad Pitt are thought to have expressed an interest in the modern Utopia, on the Cotswold Water Park.

Orchid House, modelled on a Bee Orchid, recently sold for £7.2m to a mystery buyer.

Developer Jeremy Paxton said: "Beaver kits can swim, but it may take them a month or more to figure out how to hold their breath and swim underwater. When they get tired, they catch a ride on their mother's back.

"Lower Mill residents hope to witness all this activity and more, yards from human habitation.

"It's an exciting development on an estate which, through careful management of hedges and woodland over many years, now serves as home to more than 13 classic English songbirds."