German holocaust denier Ursula Haverbeck has been sentenced to two and a half years in prison after the country's highest court ruled that denying the mass murder of Jews during Nazi Germany is not covered by the right to free speech and "threatens public peace".

Haverbeck, who is known as the 'Nazi-Grandma', was convicted last May for publishing a series of articles in which she insisted that the Holocaust and murder of Jews at Auschwitz concentration camp in German occupied Poland are not "historically proven".

In Germany, denying the Holocaust constitutes a crime of incitement to hatred and carries a prison sentence of up to five years.

The 89-year old went to Germany's constitutional court to appeal her sentence, claiming that her statements fall under the country's right to free speech, which is protected by law.

But in their ruling, the high court judges found that the right to free speech does not protect the denial of the Holocaust.

"The dissemination of untrue and deliberately false statements of fact can not contribute to the development of public opinion and thus do not fall in the remits of protection for free speech", the judges wrote in a statement.

"The denial of the Nazi genocide goes beyond the limits of the peacefulness of public debate and threatens public peace," they added.