Conservationists hope the old balls will make perfect nest houses to boost numbers living at Slimbridge Wetland Centre in Gloucestershire.

They have been put in an area of the centre for mammals called Back From The Brink.

John Crooks, mammal manager at the Wildfowl and Wetlands Trust (WWT) site, said: "We are hoping it will be a case of game, set and mouse.

"I have cut small penny-sized holes in the balls and put straw inside and they seem to be taking very well to them.

"We have about 30 in our collection here and we are hoping that the tennis balls will make them feel secure so that they breed well this year.

"A few years ago numbers of the mice in the wild did fall but they are starting to make a comeback as many populations have moved to new nesting sites such as hedgerows and wetlands."

The mice, which only measure about two inches in length when fully grown, are the smallest rodents in the UK.

Their breeding season is from May to September and they tend to have litters of four to six young. In the wild they weave circular nests out of grasses and attach them to stems high above the ground.