Italian Prime Minister Matteo Renzi has announced his resignation, saying he takes full responsibility for the "extraordinarily clear defeat" of his package of constitutional reforms in Sunday's referendum.

With more than 90 per cent of the results in, the "No" campaign led with almost 60 per cent of the vote to slightly over 40 per cent for "Yes", on course to achieve the upper estimate predicted by exit polls. 

Mr Renzi had conflated his centrist leadership with a "Yes" vote and promised to step down if he lost, and called a news conference in Rome as the result became clear. 

Addressing the nation at the Palazzo Chigi on Sunday night, Mr Renzi declared that his "experience of government finishes here". 

"We tried, we gave Italians a chance to change but we didn't make it," he said. "We wanted to win not to take part in the competition.

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"I lost. I can admit it and I am sorry. I was not able to lead you to the victory. 

"Good luck to us all," he concluded.