Cats should be kept on a leash and strays should be kept off the streets "by any means necessary" due to the danger they pose to wildlife, a scientist has claimed.

Dr Peter Marra, head of Smithsonian Migratory Bird Center at Washington DC's National Zoo, said that free-roaming cats are responsible for killing billions of birds worldwide.

He said pet cats should be kept on a leash or held in catios (garden enclosures for felines) and strays should be euthanised if they can't be re-homed.

The author of Cat Wars: The Devastating Consequences of a Cuddly Killer said that the popular pet is responsible for killing off 63 species of mammals, birds and reptiles.

Speaking to BBC Radio 4's Today programme, he said: "It's not good for cats to put them outside, it is inhumane to let them roam outside where they can get hit by cars or mauled by dogs or pick up diseases.

"Cats that are owned that are allowed to roam outside - that needs to stop.

"Stray cats should be captured and unfortunately if they can't be adopted, they will need to be euthanised."

Dr Marra's book opens with the story of Tibbles, a pet cat blamed for the extinction of the rare Stephens Island wren in New Zealand in the late 1800s.

It said that there is "mounting scientific evidence (that) confirms what many conservationists have suspected for some time - that free-ranging cats are killing birds and other animals by the billions."

But Dr John Bradshaw, an Anthrozoologist at Bristol University said that cats are not serious predators and target creatures that are weak and dying.

"You can make all sorts of stories out of what sounds like horrible numbers, millions of birds being killed," he said. "The way that birds breed, small birds anyway, is that 80% of them have to die every year or we will be knee deep in them. It's just the way they are. Billions of birds die every year."