Armed only with the number of a tattoo inked onto a four-year-old by the Nazis, Facebook users are hoping to help a Holocaust survivor track down his long-lost twin brother.

Menachem Bodner was four years old when he escaped Auschwitz, where he was in the clutches of the Nazi doctor known as the Angel of Death, Joseph Mengele, who remains infamous for his brutal experiments on twins.

Although Mr Bodner recalls little of the camp or his early life in Hungary, when he was known as Elias Gottesmann, he has long believed he had a twin. With the help of Israeli genealogist Ayana KimRon he has discovered Nazi records that prove this to be the case — and reveal that against the odds, both boys survived.

He is now using Facebook to discover what happened to Jeno, known as Jolli, after the camp was liberated by the Soviet army.

Already, 75,000 people have shared a photograph of Mr Bodner at age five, captioned “child survivor A7734” — a tattoo number one digit away from that inked onto Elias’s arm.

According to Ms KimRon, clues as to Jolli’s fate “lead to possible adoption by a Christian family, then to the USA”.