The court, which had been tasked to rule on Renzi's 2015 electoral reform, struck down a provision for a run-off vote for the lower house, saying it should be held in just one round. The rest of the law was left largely intact and the court ruled that it can be applied immediately.

In a written ruling, the court rejected a core part of the original law, which had envisaged a run-off ballot in future national elections, saying instead that the vote should be held in just one round. However, it endorsed a clause that would award an automatic parliamentary majority to any party that wins 40 percent of the vote. No opinion polls put any of Italy's plethora of parties anywhere near 40 percent at present.

That means the new system will almost certainly lead to a coalition government, something that may benefit traditional parties and penalize the main opposition 5-Star Movement, which has always refused to form alliances.

The court said the amended law could be used immediately if elections were called.