Police are investigating reports that a fox attacked a baby in its cot and bit the child's finger off.
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The one-month-old baby boy was rushed to hospital after the incident at the family's home in Lewisham, southeast London.
The Mail on Sunday reported that the fox went into the child's bedroom, dragged him from his cot and bit his hand.
The newspaper said the child's mother was alerted by his screaming and rushed into the room to find her son's hand lodged "halfway down the animal's throat".
Surgeons were able to reattach the finger and the infant is said to be recovering well.
A Metropolitan Police spokesman said: "We were called at 1638 on Wednesday 6 February by staff at St Thomas's Hospital to reports a baby boy who had been admitted to hospital after being attacked by a fox.
"Police attended to find a four-week-old baby with a hand injury. The baby was admitted to hospital after the attack at its home address in Lewisham.
Police attended to find a four-week-old baby with a hand injury. Inquiries continue. Met Police
The mayor of London, Boris Johnson, who campaigned against the 2005 ban on fox hunting while an MP, told BBC News: "They may appear cuddly and romantic but foxes are also a pest and a menace, particularly in our cities.
"This must serve as a wake-up call to London's borough leaders, who are responsible for pest control.
"They must come together, study the data, try to understand why this is becoming such a problem and act quickly to sort it out."
An RSPCA spokeswoman said: "It's extremely unusual for foxes to attack young children or anyone.
"It's not typical fox behaviour at all. Foxes will come closer to a house if there are food sources. Then they can become quite bold, but they usually do back off and run away when there's people around."
A council spokesperson said:
"Although rare, fox attacks can be devastating as this tragic case shows and our thoughts go out to the family at this difficult time.
"Local authorities and animal welfare organisations have been advising residents on ways to make their homes and back gardens less attractive to foxes. These can include repellents and making sure that waste food is always disposed of in secure, lidded bins and containers.
"Residents should always be careful when leaving doors and windows open. We must all do this together for it to be effective in discouraging foxes from our neighbourhoods."
Other fox attacks on children have been reported in the media in recent years, but some sceptics have questioned the truth of the stories.
Last year, wildlife presenter Chris Packham said he did not believe reports of people getting attacked by the animals.