Italians will vote in new general elections in 2018 no matter how a referendum on constitutional reform turns out later this year, Italian Prime Minister Matteo Renzi said on Sunday.

Renzi, who came to power two years ago by ousting grand coalition-leader Enrico Letta, has staked his political future on winning the referendum which he says is crucial to more stable and stronger government.

Asked in an interview before an audience at an outdoor festival in Marina di Pietrasanta in Tuscany whether elections would be held in 2018 whatever the outcome of the popular vote, Renzi said "yes".

But the 41-year-old indicated he would nonetheless step down from office if he lost the referendum.

"If the no [vote] wins I have already said what I will be doing," he said. Renzi has previously commented he would resign.

The referendum, the date of which has not yet been announced, seeks to do away with a parliamentary system in which the upper and lower houses have equal powers, effectively abolishing the Senate as an elected chamber and sharply reducing its ability to veto legislation.

Critics fear the proposed reform could give excessive powers to the government and its leader.

Renzi reiterated it had been a mistake to personalize a referendum in which he promised to resign if he failed to convince voters to support the need for constitutional change.

"I made a mistake and because of this the whole thing has become a debate about everything," he said.

No government has completed a full term in Italy since World War Two, making it difficult to reform an economy where debt-to-output is second only to Greece in the euro zone.

Asked about the next budget Renzi said common sense was needed but added taxes needed to keep coming down.

"For Italians cutting taxes is fundamental," he said.