A fox left a one-month-old baby boy with a serious hand injury after creeping into his bedroom and dragging him from his cot.

Police said they were investigating the incident in Bromley, south east London, in which the animal tore the infant's finger off.

The child's screams alerted his mother, who rushed to his room to see his hand lodged "halfway down the animal's throat", according to the Mail on Sunday. She is said to have kicked the fox until it let go.

Surgeons managed to reattach the finger in an "extremely difficult" three-hour operation, the newspaper reported. The child, who also suffered puncture wounds to his face, was said to be recovering well.

The attack happened after the fox entered the house through an open back door, which was apparently awaiting repair by the council.

A Metropolitan police spokesman said: "We were called at 16.38 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 Bromley."

An RSPCA spokeswoman said the only reason a fox would attack is due to fear. She added: "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 fox attack in June 2010 left nine-month-old twins Lola and Isabella Koupparis scarred for life. The girls' mother, Pauline, described the mauling at their east London home as a "living nightmare".