The other members of the SLMC who crossed over were SLMC General Secretary Hassan Ali, Basheer Sagudawood, and Faisal Kassim, who is also a government deputy minister.
Hakeem said he would be giving up his portfolio as minister of posts and telecommunications.
Sri Lankan leaders in recent years have been overly generous with public money in doling out ministerial portfolios and the perks and privileges that go with them to buy the loyalty of opposition politicians.
Such farcical political practices prompted Britain's outgoing envoy in Colombo Dominick Chilcott to mock the practice of crossovers, saying in a speech Monday it was an open secret lawmakers have been made ministers to buy their loyalty to the government.
Updated
Rauf Hakeem, leader of the Sri Lanka Muslim Congress, crossed over to the opposition benches along with three others in the afternoon.
Their defection comes ahead of a crucial final vote in parliament on Friday on the government's 2008 bu