A suspected former Nazi guard who acted as a witness in the trial of John Demjanjuk may be prosecuted for war crimes.

According to Spiegel news magazine German prosecutors are considering charging “Alex N” for involvement in killing Jewish prisoners at Treblinka concentration camp during the Holocaust.

The 93-year-old was born in the Ukraine but gained German citizenship in 1991.

Researchers investigating the case said other camp guards called having heard Alex N boast about shooting Jews.

The case came about after he testified at the trial of Mr Demjanjuk, who is accused of assisting with the massacre 27,900 Jews at Sobibor in 1943.

Mr Demjanjuk went on trial for war crimes in Munich in November 2009, but complaints of health problems have caused proceedings to be delayed.

A judge has now ordered him to appear in court despite this.

The Munich state prosecutor has launched an investigation into the Alex N case and will now decide whether to bring charges.

If they do, he will be the third alleged SS guard to face prosecution in the past year.