Urban foxes are a common sight in many parts of London, hunting and scavenging at night from dens in parks and empty lots. They are generally timid and not considered dangerous, and attacks on humans are virtually unknown.
Police said the attack on the 9-month-old sisters occurred when the fox entered the east London house Saturday night. London experienced unseasonably warm temperatures over the weekend.
Police described the girls' condition as serious but stable.
Neighbor Michael Parra said Monday that police had warned residents not to keep their doors open. Parra said residents "see a lot of foxes around here. They're always in our garden."
Hackney Council said it had never received a complaint about a fox attacking a human and "all the expert advice is that incidents like this are incredibly rare."
John Bryant, an expert in pest control, told the BBC that foxes tend to avoid people.
"They will walk into houses, walk round, mess on the bathroom floor and sometimes sleep on the bed if people are not around," he said, adding that he could see no reason for an attack "unless it jumped into the (crib) and then found itself with squirming children underneath it and couldn't get out.
"It just doesn't make any sense to me."
The incident may remind Atlanta, Georgia residents of a recent incident in a parking lot, when a security camera captured a local resident trying to fend off a rabid fox that attacked him. Click on the player below to watch the video.