British people have always thought of foxes as harmless, cute creatures.

Many enjoy the occasional sight of a wild fox loping across their garden. Puppet Basil Brush became a popular television character with generations of children.

But the fox's reputation as an unobtrusive cohabitant of human cities took a severe knock yesterday after two baby girls were taken to hospital - having apparently been bitten by an animal who wandered into a three-storey Victorian house in Hackney, through patio doors left open because of the heat.

The nine-month-old twins, Lola and Isabella Koupparis, were sleeping in their cots while their parents watched television downstairs with the twins' four-year-old brother.

"We were watching Britain's Got Talent, it had just finished," the twins' mother, Pauline Koupparis told BBC London yesterday. "We heard the girls cry. I went up the stairs. As soon as I got to the second step I thought it's a funny cry, it's not a normal cry. It was quite muffled but very pained.

"I went into the room and I saw some blood on Isabella's cot. I thought she'd had a nose bleed. I put on the light and I saw the fox. It wasn't even scared of me. It just stared me straight in the eye. I started screaming.

Then I realised that Lola was also covered in blood. My husband came running up - by this stage we were both screaming hysterically - and the fox didn't even move. My husband lunged at it a few times and it just moved a few inches each time."

One girl, who slept on her back, had been bitten on the face. The other, who was asleep on her stomach, had been bitten on the arm. Both were taken to hospital. Isabella, who was bitten on the arm, is in intensive care. Police described the condition of both twins as "serious but stable".

Asked how her daughters were faring yesterday, Mrs Koupparis said: "One is really good and one is not so good." She added: "It's a living nightmare. It's something I would never have expected to happen - let alone to us and my beautiful girls."

The day after the attack, pest control officers from Hackney Council set traps in the Koupparis's garden and caught a fox, which was humanely put down - but ironically, they don't even know whether it was the same animal which attacked the twin girls.

The news came as a shock to people who have spent their working lives studying the behaviour of foxes. They said it is extremely rare for one of the animals to attack a human, although there have been other occasional reports of similar incidents.

In Edinburgh five years ago, an 88-year-old woman was bitten by a fox when she went out into her garden late in the evening to feed her cats. In 2002, a fox reportedly crept into a house in Dartford, Kent, and bit a 14-week-old boy on the head.

John Bryant, a pest-control consultant who specialises in foxes, told BBC's Today programme that he had dealt with only two alleged fox attacks in 40 years. In one case the "fox" proved to be an Alsatian dog, and the other was a cat. He described the attack on the baby girls as a "freakish event".

"It's not, in my experience, fox behaviour," he added. "If it was a fox, it must have been a cub. There are thousands of three-month-old teenage cubs now wandering around, beginning to explore their parents' territory. They will walk into houses, walk round, mess on the bathroom floor and sometimes sleep on the bed if people are not around."

Martin Hemmington, founder of the rescue charity the National Fox Welfare Society, said urban foxes will usually avoid contact with humans. "It takes quite a lot of effort to catch them," he said. "Walking into people's houses is not commonplace and they would never go in with the intention of attacking someone. I can only imagine the fox has found itself in a situation and it has become distressed and panicked. They are wild animals and will bite if cornered."

But Ricky Clark, a rat catcher who runs the London-based firm Environ Pest Control, warned: "Yes, it's unusual, but it will happen again. We have 25,000-30,000 foxes in London alone, and they are losing their fear of humans. I've been getting calls from people who say, `We've just had a fox come through our cat flap and it's in the sitting room'. I've caught foxes in kitchens, in basements, even in nightclubs. These aren't the cuddly little red furry animals some people think they are. It's not Basil Brush we're dealing with here. These are feral animals, full of diseases.

"In some circumstances, their faeces are more dangerous than rat droppings. They need to be controlled, and people should not feed them."

Hackney Council yesterday urged people not to feed foxes, either deliberately or unintentionally, by leaving food out in unprotected rubbish bags.