Factbox - Impeachment of a President: How it works in Italy

ROME (Reuters) - The head of Italy's 5-Star Movement, Luigi Di Maio, has raised the possibility of an attempt to impeach President Sergio Mattarella, a process that has been initiated three times in the past against Italian heads of state but has never succeeded.

Di Maio has so far failed to win broad support for his call, made after Mattarella put a veto on the economy minister that his anti-establishment party and the far-right League had put forward for a coalition government. League chief Matteo Salvini said he "didn't want to talk about it".

But even if Di Maio fails, the decision has created a rare and aggressive backlash against the head of state - a highly-respected figure in Italian politics - with some citizens questioning whether he is really protecting national interests.

The lengthy and elaborate impeachment procedure, with a final decision resting in the hands of Italy's highest court, would not be compatible with a call for swift repeat elections to break the political deadlock.

WHAT IS THE PROCEDURE?

The process is regulated by article 90 of the Italian constitution. Under this, the president can be impeached "for high treason or attacks against the constitution".

The impeachment motion must be filed to the speaker of the Chamber of Deputies, the lower house of parliament, and it is then reviewed and investigated by a special committee made up of 20 lawmakers, representing all parties in parliament.

The committee has a maximum of eight months before it must decide whether to reject the motion or send it to parliament for a vote in a joint session of the upper and lower houses.

Such a vote is held in a secret ballot, with the motion requiring a simple majority to be approved.

Parliament still does not have a final say. Once approved, this rests with the 15 judges of Italy's highest court - the Constitutional Court - expanded to include 16 people drawn up separately from outside parliament.

The Constitutional Court proceedings are carried out according to its normal process, with hearings, debates and witnesses, and at the end the court hands down its decision, which cannot be appealed.

HOW LIKELY IS IT?

No Italian president has even been impeached, although the procedure was initiated for three heads of state in the past.

In 1978 President Giovanni Leone stepped down when the Italian Communist Party announced its intention to start the procedure, while in 1991 the special committee rejected the request to impeach Giovanni Cossiga.

The last one was filed in 2014 by the 5-Star Movement against Mattarella's predecessor, Giorgio Napolitano, but it was also rejected by the special committee.

(Reporting by Giulia Segreti; editing by David Stamp)