ROME (Reuters) - Italy's highest court on Tuesday confirmed an acquittal for former Prime Minister Silvio Berlusconi on charges of abuse of office and paying for sex with a minor, giving him a boost as he is trying to hold onto his role as a conservative leader.

At the first trial in 2013, Berlusconi, 78, was convicted and sentenced to seven years in prison. But that ruling was overturned by an appeals court in Milan that acquitted him last year.

The Rome court, in a ruling that is now definitive, rejected an appeal by Milan prosecutors to overturn the acquittal and to hold a new trial.

Berlusconi was accused of paying for sex with former teenage nightclub dancer Kharima El Mahroug, known by her stage name "Ruby the Heartstealer", during "bunga bunga" erotic parties at his palatial home near Milan when he was prime minister in 2010.

He was also charged with abusing his authority to get El Mahroug, who was 17 at the time, released from police custody over unrelated theft accusations.

The trial was the most sensational Berlusconi faced. It mesmerized Italy with its lurid accounts of sex parties. A stream of would-be starlets on his TV channels took the stand as witnesses.

Tuesday's ruling came at a time when the conservative leader is struggling to hold together his Forza Italia party and maintain a front-line role in Italian politics.

Last month Berlusconi pulled out of a pact with Prime Minister Renzi over reform of Italy's voting system and political apparatus, complaining that Renzi had excluded him from the choice of who should become the new head of state.

On Tuesday Forza Italia voted against Renzi's political reforms in the Chamber of Deputies, but 18 party deputies wrote a public letter of dissent. They lamented a lack of internal party democracy and said they had only obeyed Berlusconi's instructions out of loyalty to their leader.

Forza Italia has steadily lost support since Berlusconi was convicted for tax fraud in 2013. As a result of that sentence he lost his seat in the Senate and was barred from holding any public office.

According to most polls the party now has only 13-14 percent of the vote. That puts it behind Renzi's Democratic Party, the anti-establishment 5-Star Movement and the anti-immigrant Northern League whose leader Matteo Salvini seems to be eclipsing Berlusconi as the prominent figure on the Italian right.