Austrian prosecutors have filed an appeal against the three-year prison sentence handed to the British historian David Irving, arguing that he escaped too lightly for the crime of Holocaust denial.

Irving was left stunned and open-mouthed when the sentence was handed down after a one-day trial in a Vienna court yesterday.

After entering a guilty plea and publicly accepting that he had made a mistake when denying existence of gas chambers at Auschwitz, Irving had clearly expected a more lenient punishment over two speeches made to Austrian neo-Nazis in 1989.

Irving's defence lawyer, Elmar Kresbach, has already appealed against the severity of the sentence. But Walter Geyer, spokesman for the public prosecutor's office in Vienna, said that prosecutors in the trial filed their own appeal today.

"The public prosecutor believes the ruling was too lenient in light of a possible sentence of up to ten years and Irving’s special importance to rightwing radicals," Herr Geyer said.

Herr Kresbach was not available for comment this afternoon, but has said that Irving was unlikely to serve the full three-year term because of various factors, including his age.

Irving, whose reputation as a historical researcher was destroyed in a High Court libel trial in 2000, is the author of nearly 30 books, including Hitler’s War, which challenges the extent of the Holocaust and Adolf Hitler's knowledge of it. He has also contended most of those who died at concentration camps such as Auschwitz succumbed to diseases such as typhus rather than execution.

In yesterday's trial, Irving acknowledged denying in 1989 that there was an organised Nazi genocide against Jews and that gas chambers existed, but said he changed his mind in 1991 after reading the personal files of Adolf Eichmann, the chief organiser of the Holocaust. "The Nazis did murder millions of Jews," he told the court.

Herr Geyer said that the prosecutors had argued that Irving had only pretended to moderate his views in order to reduce his sentence. That echoed a declaration by the presiding judge in the case, Peter Liebetreu, who said that Irving's expressions of regret were "considered to be mere lip service to the law".

The appeals will be heard by the Austrian Supreme Court, although probably not until the second half of this year.