A German serial killer who murdered five female hitchhikers accidentally killed himself while performing a strange solo sex act in his prison cell over the weekend, reports claim.

Egidius Schiffer, dubbed the “Aachen Strangler,” gave himself a fatal electric shock after tying a cable from a lamp around his penis and nipples, police said.

He was still tied up when guards at Bochum prison, in the North Rhine-Westphalia state, opened his room Monday morning, the Times reported.

Prison spokeswoman Candida Tunkel said: “He removed a cable from his bedside table lamp. Then he wound it around his nipples and his penis and stuck the end in a power socket.”

A post-mortem established that Schiffer, 62, died of heart failure as a result of an electric current flowing through his chest.

Cops said they did not suspect foul play and his lawyer claimed the murderer was not suicidal.

Schiffer was sentenced to life in 2008 for a string of grisly murders committed between 1983 and 1990.

The authorities used DNA evidence to convict him 18 years after his final murder.

He picked up his victims, one of whom was just 15, as they tried to hitch rides home from nightclubs before handcuffing and tying them up.

The monster raped three of the women and buried them in the woods.

The former insurance salesman was arrested back in 2007 while stealing scrap metal.

He decided to give a DNA sample voluntarily that eventually connected him to the unsolved murders.

Schiffer initially confessed before withdrawing his testimony, claiming he was sadomasochistic and the thought of being locked up excited him.