Austrian prosecutors are seeking to lengthen the three-year prison term given to British historian David Irving for denying the Holocaust.
David Irving said his views had changed since his 1989 remarks
Irving is appealing for a reduction in the jail term, but prosecutors say the sentence is too lenient and have filed their own appeal.
Irving pleaded guilty in his one-day trial in Vienna on Monday.
The charges arose from comments he made in Austria in 1989 denying the existence of gas chambers at Auschwitz.
Austria is one of 11 countries with laws against denying the Holocaust.
'Importance to radicals'
The Austrian state prosecutor's office said it believed Irving's sentence was too lenient in light of a possible sentence of up to 10 years.
The prosecutor also deemed the sentence too light because of "Irving's special importance to right-wing radicals", a spokesman for the office said.
In his trial, the 67-year-old historian admitted that in 1989 he had denied that Nazi Germany had killed millions of Jews.
He said this was what he believed, until he later saw the personal files of Adolf Eichmann, the chief organiser of the Holocaust.
"I said that then based on my knowledge at the time, but by 1991 when I came across the Eichmann papers, I wasn't saying that anymore and I wouldn't say that now," Irving told the court.
"The Nazis did murder millions of Jews."
In the past, he had claimed that Adolf Hitler knew little, if anything, about the Holocaust, and that the gas chambers were a hoax.
Irving's lawyer has said his client is unlikely to serve the full three-year term because of various factors, including his age.