Italian President Giorgio Napolitano, who led the country through one of its stormiest political periods, resigned on Wednesday, opening a phase of political uncertainty that will test the strength of the young government of Prime Minister Matteo Renzi.

The early end of Mr. Napolitano's unprecedented second term deprives Mr. Renzi of a crucial ally in his battle to push institutional and economic changes for a country that suffers from poor competitiveness and is locked in a triple-dip recession. Mr. Napolitano has often helped prod Italy's quarrelsome parties to support Mr. Renzi's ambitious agenda.

In his traditional end-of-year speech, the Italian president, who will turn 90 in June, said he had decided to end his second mandate ahead of time, due to his age and increasing "signs of fatigue," which made it increasingly difficult for him to fulfill his institutional duties.

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