ROME - Italian Prime Minister Matteo Renzi said Tuesday he is confident the Italian people will "vote with their heads not their hearts" in a Dec 4 referendum on his government's constitutional reform law.

The measure has already been approved by parliament, but must be confirmed -- or rejected -- by a popular vote because it amends some parts of the Constitution.

"Italians are wise and know how to vote with their heads not their hearts," Renzi said.

"I urge all citizens to concentrate on the referendum question and not on external issues" such as the stock market, the center-left prime minister said.

"The spread and the markets are secondary," he said.

The spread between Italian 10-year bonds and their German counterparts is an indicator of investor confidence and the country's borrowing costs, and has been climbing as the referendum date draws closer. The higher the spread, the less the investor confidence.

On Tuesday, the spread closed at 172 basis points after soaring above 190 points last week, and has been hovering at two-year highs on fears a "No" referendum victory could bring about Renzi's resignation, which would likely be followed by political and economic instability.

Also on Tuesday, the Italian prime minister said voters who are still undecided about whether to accept or reject his landmark reform "are deciding as we speak", and that he believes supporters of opposition parties will vote in favor of his reform on its merits.

"All I ask is that you keep in mind that this opportunity won't come back when you enter the voting booth," Renzi said.

"Governments come and go, reforms don't come back," he stressed.