A Greek court has reversed its decision to give parole to the leader of the neo-Nazi Golden Dawn party Nikos Michaloliakos, a judicial source said on Monday.

The 66-year-old, who was granted early release on May 2, was sentenced to 13 and a half years in prison in 2020 as the presumed head of a criminal organisation that attacked immigrants and political opponents.

Crimes imputed to the group included the 2013 murders of an anti-fascist rapper and a Pakistani migrant, as well as the beatings of Egyptian fishermen and Communist trade unionists.

Examining an appeal by the deputy prosecutor in the city of Lamia, the judicial council ruled that Michaloliakos had not shown good behaviour and should return to prison.

Michaloliakos had published writings about his conviction which proved he had not atoned for his previous behaviour, the source told AFP.

When he was given conditional release in May, Michaloliakos was ordered not to leave the Athens area and was required to report to a local police station once a month.

The mathematician and Holocaust denier was also banned from contacting with other people convicted in the case.

His release sparked outcry from the left and human rights organisations.

Golden Dawn, a xenophobic and anti-Semitic organisation created by Michaloliakos, was for decades a fringe party until the country's 2010 debt crisis.

The group then capitalised on public anger over immigration and austerity cuts, entering parliament for the first time in 2012.

At the height of its influence, it was the country's third biggest party.