BERLIN (Reuters) - Germany's highest court has rejected an appeal filed by a man known as the "bookkeeper of Auschwitz", who was sentenced last year to four years in prison for his role in the murder of 300,000 people at the Nazi death camp.

It said it rejected the appeal of Oskar Groening, 95, who was convicted in July 2015 of aiding and abetting the murders, as well as appeals filed by several other people who argued that Groening should have been convicted of the more serious charge of being an "accomplice" to murders.

"The conviction is therefore now legally binding," the Federal Court of Justice said in a statement. It said it took the decision in September but only made it public on Monday.

German Justice Minister Heiko Maas welcomed the decision. "It is never too late for justice," Maas said in a statement. "When it comes to the legal processing of Auschwitz there can also never be an end."

Groening, a former Nazi SS officer, did not kill anyone himself while working at the camp in Nazi-occupied Poland, but the lower court ruled that he helped support those responsible for mass murder through various actions, including by sorting bank notes seized from trainloads of arriving Jews.

Hans Holtermann, Groening's attorney, told Reuters there was no possibility of a further appeal, but he was studying whether to file a complaint with Germany's constitutional court given that Groening had been under investigation since 1977.

"We think that should have been taken into consideration," he said.

The trial went to the heart of the question of whether people who were minor participants in the Nazi atrocities, but did not actively participate in the killing of 6 million Jews during the Holocaust, were themselves guilty.

The now frail Groening admitted moral guilt but said it was up to the court to decide whether he was legally guilty. During the trial he said he could only ask God to forgive him as he was not entitled to ask this of victims of the Holocaust.

Legal experts said the court's decision could set a precedent for the case of Reinhold Hanning, another former SS officer who was convicted in June for his role in the murder of at least 170,000 people at the concentration camp.

The Federal Court of Justice said it had not yet received any appeal relating to his case. A spokeswoman said an appeal was likely still being considered by federal prosecutors.

(Reporting by Andrea Shalal in Berlin and Elke Ahlswede in Frankfurt; Editing by Alison Williams)