Some of the defectors have been rewarded with cabinet portfolios, and there were several others waiting to join the administration, the spokesman said.
Key to stability
The defections left the government just shy of a parliamentary majority seen as key to political stability amid a new chapter in a two-decade civil war with Tamil Tiger rebels, and could also derail a cross-party push to find a political solution to the conflict.
Six members of the Sri Lanka Muslim Congress, the island's main Muslim party, also joined the government.
Analysts warned that the UNP cross-overs had effectively ruptured the opposition party at a time when it was working with the government to build consensus on how to end a war that many fear is set to escalate.
Rajapakse's Sri Lanka Freedom Party and the opposition UNP in October agreed to work together to hammer out a political solution to the long-running Tamil separatist conflict.
Radical constitutional changes require a two-thirds majority in the 225-member assembly.
However, Tissa Attanayake, the UNP secretary, said the deal would be over the moment their members were accommodated on the government side.
He said: "From the time the defectors are given portfolios and taken to the government side, our MoU (memorandum of understanding) will be over."
Sri Lanka's key international backers, including the US, European Union, Japan and India, had wanted the two main majority Sinhalese parties to build on their October deal to offer a consensus plan to Tamil Tigers.
Foreign aid donors had asked Sri Lanka's leaders to consolidate the deal and resume negotiations with the Tigers in line with a Norwegian-backed peace initiative. The last round of peace talks in October ended in failure.