Finland's outgoing prime minister says he will step down as head of the Center Party, which had one of its worst performances in decades in Sunday parliamentary election.

Juha Sipila tweeted Tuesday "the election result leaves me with no choices," referring to the centrist party's meager 13.8% support and loss of 18 parliamentary seats in Finland's 200-seat legislature.

Sipila is due to step down at an extraordinary party congress on Sept. 7.

He has chaired the Center Party, a major political establishment, since 2012, and been the Nordic country's prime minister since 2015. Sipila continued heading a caretaker government after his three-party coalition resigned in March,

The center-left Social Democratic Party emerged as Finland's largest party with a tight win over the populist Finns Party in the election. Only 0.2 percentage points separate them.