President Mahinda Rajapakse swore in Sunday a 53-member cabinet, by far the largest since independence from Britain in 1948.
He accommodated 10 defectors in the cabinet and made eight others junior ministers, but sidelined some of his own party stalwarts.
"In the short term, the government will enjoy a parliamentary majority, but this does not mean stability," said Sunanda Deshapriya, director at the Centre for Policy Alternatives (CPA) think tank.
He said the serious ideological differences between those who joined the government Sunday and the rest of the ruling Sri Lanka Freedom Party could lead to an uneasy cohabitation.
The rump of the opposition was also expected to step up agitation.
"There is nothing to indicate that the new cabinet will shift gear and pay more attention to the peace process," Deshapriya said.
"The government still believes it can crush the Tigers and then go for (peace) talks."
As well as the record number of cabinet ministers, 33 were named non-c