By Crispian Balmer

ROME (Reuters) - Italy's highest court will rule on Wednesday on the legality of its latest electoral legislation, a court spokesman said on Tuesday, a decision likely to influence the timing of the next election.

An unambiguous ruling offering a simple solution to Italy's electoral tangle could open the way for an early ballot by June. A more nuanced, convoluted reading would almost certainly leave parliament in place until the legislature ends in early 2018.

Thirteen Constitutional Court judges heard arguments on Tuesday for and against the 2015 law, which was drawn up by then-prime minister Matteo Renzi. They will deliver their verdict at around 12.30 p.m. (1130 GMT) on Wednesday.

The court is widely expected to decree that parts of the legislation, which is known as the Italicum, are invalid.

The law relates only to the lower house of parliament. It envisaged a ballot staged over two rounds that guaranteed a big majority to the winning party while granting party bosses wide powers to handpick their preferred candidates.

Renzi said the Italicum would bring political stability to Italy, while critics complained that it concentrated too much power in the hands of the winning party and did not allow voters to directly choose their representatives.

A recent spate of opinion polls have suggested that the two-round voting system would benefit Italy's main opposition party, the anti-establishment 5-Star Movement, which wants a referendum on continued membership of the euro currency.

Many legal experts expect the court to endorse a one-round ballot, ruling that a run-off vote between the top two parties is invalid in a multi-polar political system.

NO CONSENSUS

"If the court intervenes to change the electoral law, removing the second round and essentially leaving a proportional system in place, then the likelihood of an election this year is very high," said Anna Ascani, a lawmaker close to Renzi.

"But there's no consensus in parliament on what kind of electoral law to write should the court only make suggestions to change the current law," she told Reuters, saying such a scenario would make a snap vote in 2017 highly unlikely.

Renzi resigned last month after a crushing defeat in a referendum on his plans to overhaul the constitution, but he remains leader of the ruling Democratic Party (PD) and is pushing for elections to allow him to return to power.

The constitutional reform was based on eliminating the upper house Senate as a directly elected chamber and when Renzi drew up his electoral reform he only had the lower chamber in mind, confident that voters would back him in the referendum.

His defeat means that different laws now govern elections in the two houses, so whatever the court rules this week the conflicting systems will have to be harmonized by parliament.

The new premier, Paolo Gentiloni, who is close to Renzi, said at the weekend he was confident a new election law could be in place in time for a vote in the summer or autumn.

Spooked by the chance of a 5-Star victory, many politicians in both mainstream center-left and center-right parties are calling for the introduction of a proportional representation system (PR), which would almost certainly necessitate coalition-building - something 5-Star has always refused to countenance.

However, there are numerous PR options and politicians unsure of winning re-election at the next vote could yet string out discussions to ensure that parliament survives for the full legislature, thereby boosting their pension rights.

(Reporting by Crispian Balmer; Editing by Mark Heinrich)