Italians are voting Sunday on whether to limit offshore drilling rights for oil and gas, amid expectations that the referendum, backed by environmental groups and opposition parties, was going to be declared void due to low turnout.
In Italy, referendums can be held to scrap pieces of legislation, but their outcome is valid only if at least 50 per cent of eligible voters take part.
Polling stations opened at 7 am (0500 GMT) and were due to close at 11 pm. Critical turnout data is usually announced shortly after voting is over, while it takes several hours for final results to be declared.
The vote is on whether energy companies should be able to keep extracting oil and gas from more than 40 offshore platforms within 12 nautical miles (22 kilometers) of the coast until reserves run out, or stop once current licences expire over the 2018-2034 period.
The issue is quite technical.
Backers of the referendum say it would limit fossil fuel extraction in Italy - which would continue on land and beyond 12 nautical sea miles - reducing an activity representing a an environmental hazard and covering only a tiny fraction of Italy's energy needs.
Prime Minister Matteo Renzi has urged voters to boycott the poll and abstain, arguing that curbing drilling rights would threaten 11,000 energy industry jobs and increase Italy's dependency on energy imports from Arab countries and Russia.
Politically, the vote is also seen as a trial run for another referendum due in mid-October, which will see Renzi square off against the same opposition parties over major constitutional reforms he has recently pushed through parliament.