Italy's Matteo Renzi has joined his fellow Italians to vote in a constitutional referendum on which the reformist Prime Minister has staked his future.

Polling stations opened at 7 am local time and are scheduled to close at 11 pm for some 46 million eligible voters.  The referendum will decide the political future of Renzi, who has promised to resign if he loses.

Renzi's plan is to drastically reduce the role of the upper house Senate and claw back powers from regional authorities.

Financial markets and Europe's politicians fear victory for the opposition "No" camp could trigger political instability and renewed turmoil for Italy's battered banks.  Previous surveys put the "No" camp ahead.