(CNN) A former Nazi officer known as "the bookkeeper of Auschwitz" has been told he must serve out his four-year prison sentence, despite lodging an appeal for clemency.

Oskar Groening's plea for mercy was denied by German prosecutors in Lueneberg on Wednesday, one day after his appeal was made public.

Groening, 96, was sentenced in 2015 after being found guilty of being an accessory to the murder of 300,000 people at the Auschwitz death camp in Nazi-occupied Poland during World War II.

He was accused of counting the cash found in the belongings of new arrivals at the camp, where more than 1 million Jews died, and sending it to Nazi headquarters in Berlin.

During the trial, Groening admitted that he was "morally complicit" in the crimes but denied that he was legally guilty.

A request by Groening's lawyers for his sentence to be suspended on the grounds of old age was rejected by the prosecutor in Hannover last August.

Groening can appeal Wednesday's decision to the Hannover Ministry of Justice, but it is not yet clear if he will do so.

According to Hannover prosecutor Kathrin Soefker, Groening will have a week to turn himself in once the Hannover Justice Ministry receives official notice from the local prosecutor. If that deadline passes, police will be sent to carry out the sentence.

Soefker told CNN that Groening remains at home and has not yet served any time in prison. Groening's lawyer declined to comment on the case.

Auschwitz concentration camp is now home to a Holocaust museum.

Door open to further trials?

The trial of the "bookkeeper" is one of a number of criminal proceedings recently carried out against former Nazis.

In 2016, Reinhold Hanning, a former SS guard at Auschwitz, was convicted of having assisted in the deaths of 170,000 people and sentenced to five years in prison.

The trial of Hubert Zafke -- then 95 and accused of being an accessory to at least 3,681 murders at the same camp -- also began in 2016, but ended in September last year after he was deemed no longer fit to stand trial due to dementia, according to Reuters.

About six million Jewish people died in Nazi concentration camps during the war. Hundreds of thousands of Roma people, homosexuals, disabled people and other persecuted minorities were also killed.

Only in the last few years have former Nazis who were not directly involved in the killings been successfully convicted.

The legal doctrine under which Nazis can be tried in Germany began to evolve with the conviction in 2011 of John Demjanjuk as an accessory to the murder of 28,000 Jews in the Sobibor death camp in Poland.

After the sentencing of Groening in 2015, Efraim Zuroff, the chief Nazi hunter for the Simon Wiesenthal Center in Los Angeles, told CNN that the conviction extended that doctrine even further and opened a door to further trials of alleged Nazi criminals.

As a guard, Demjanjuk dealt with prisoners and moved them around, but Groening had a desk job, Zuroff explained. Groening was, he once said, a mere "cog in the gears."

But that was enough to convict him, making new prosecutions more feasible, according to Zuroff.