Italian Prime Minister Matteo Renzi speaks during the last rally for a "Yes" vote in the upcoming referendum about constitutional reform, in Florence, Italy, December 2, 2016. REUTERS/Alessandro Bianchi

ROME (Reuters) - Italians started voting on Sunday in a referendum on constitutional reform, with Prime Minister Matteo Renzi promising to resign if he loses the ballot.

Polls opened at 7 a.m. (0600 GMT) and are set to close at 11 p.m. (2200 GMT). Some 51 million Italians are eligible to vote and full results are expected in the early hours of Monday.

The reform reduces the role of the upper house Senate and claws back powers from regional authorities -- measures that the government says will bring political stability to Italy. However, almost all opinion polls over the past two months have suggested that Renzi will be defeated.