His decision to change sides came as a numbers game in parliament heats up ahead of a crucial vote on the 2008 budget next week.
Opposition parties have said they will vote against the budget but the ruling coalition maintains it has enough votes in parliament to get it passed.
The government would fall if it fails to get the budget passed by parliament, and a snap election would follow.
On Wednesday, Wijeyadasa Rajapaksa of the governing Sri Lanka Freedom Party (SLFP), crossed over in parliament and sat on the opposition benches.
He said he would remain a member of the SLFP but put forward conditions for his support to pass the budget.
These include the dismissal of cabinet ministers and top bureaucrats accused of corrupt