MP Harin Fernando has decided to leave the SJB, informed political sources said.
He would function independently in Parliament without supporting any party, they added.
It is also learned that MP Manusha Nanayakkara and several others will follow suit in the future.
This will cause a major split in the main opposition party led by Sajith Premadasa.
It is also reported that they will offer conditional support to the new interim government that will be formed in Parliament in the coming days.