The United People's Freedom Alliance, led by President Maithripala Sirisena, has decided to leave the government.
UPFA General Secretary Mahinda Amaraweera said the decision had been conveyed to Speaker Karu Jayasuriya.
This marks the end of the unity government, formed in September 2015, following the last Parliamentary election.
However, ministers and deputy ministers of the UPFA, who are currently holding positions in the government are yet to step down.