BERLIN (Reuters) - The German president of the European Parliament, once compared to a Nazi concentration camp guard by former Italian prime minister Silvio Berlusconi, warned Italians on Thursday not to back the scandal-ridden media tycoon at the ballot box.

Martin Schulz is the latest in a line of German politicians to express fears about a possible Berlusconi comeback largely due to worries he will halt Rome's reform drive that has helped to lift investor confidence in the euro zone.

"Silvio Berlusconi has already sent Italy into a tailspin with irresponsible behavior in government and personal escapades," Schulz was quoted as saying in German daily Bild.

Berlusconi has been sentenced to prison for tax fraud and is on trial for having sex with an under-aged prostitute.

In quotes not printed in the paper but sent in an advance copy of the report, Schulz went on to urge Italians to vote for continued reforms.

"Much is at stake in the forthcoming elections, including making sure that the confidence built up by (Prime Minister) Mario Monti is not lost. I am very confident that Italian voters will make the right choice for their country."

There is bad blood between Berlusconi and Schulz, a Social Democrat.

In 2003, the then Italian prime minister said he would like to suggest Schulz, who had criticized his policies, for the role of a Kapo in a film on Nazi concentration camps. A Kapo was a camp inmate given privileges for supervising prison work gangs.

Berlusconi later brushed off the comment, saying he was being ironic and had referred to a television comedy series.

Merkel herself, who has supported Monti's austerity drive and has a difficult relationship with Berlusconi, has kept mum on who she would like to see lead the euro zone's third biggest economy .

Monti risked embarrassing her this week by saying Merkel did not want the centre-left Democratic Party (PD) to win, following Berlusconi's remark that Monti would join forces with the PD after the election "with Merkel's blessing".

Polls suggest the PD is likely to win control of Italy's lower house in the February 24-25 vote. Monti's centrist alliance is trailing in fourth place, but the PD may need its support to control the upper house of parliament and form a government.

(Reporting by Madeline Chambers; Editing by Stephen Brown)