Doctors were stunned after an 11-year-old schoolgirl was admitted to a hospital in China with a third arm sprouting out of her back.

The large mass was revealed to be part of an identical twin that had failed to develop and had been enveloped into the body of its sister. Further investigation found it consisted of a breast and an arm which included a shoulder blade and two fingers.

Writing in the journal BMJ Case Reports, Prof Peng said: 'Based on physical examination and imaging findings, a diagnosis of fetus in fetu was made preoperatively.'

Fetus in fetu in an  incredibly rare condition that affects only one in five million live births. It is a condition where the malformed foetus is found in the body of its twin. The foetus is found in the abdomen in 80 per cent of cases, although there have been reports of it occurring in the skull. However, this is the first time that is has been found on the back.