Sri Lankan Health Minister Maithripala Sirisena has announced that he is defecting from the party in order to stand against the President Mahinda Rajapaksa in the upcoming presidential elections.
According to the BBC, it is a major setback for the President politically ahead of the January vote.
Sirisena was one of the most powerful men in the ruling party.
Five other government members of Parliament have also announced their defection.
President Rajapaksa is seeking a third term in office after coming to power in 2005 and securing victory over Tamil Tiger rebels in 2009.