Sumathipala made the announcement at a media briefing in Colombo today.
The Sri Lanka Freedom Party member is was among 16 SLFP members who left the Government and sat with the opposition.
Sumathipala had also voted in support of a no-confidence motion on Prime Minister Ranil Wickremesinghe, which was eventually defeated in Parliament.
The United National Party had then demanded that Sumathipala resign from his post as Deputy Speaker. (Colombo Gazette)