Kadima spokesman Imri Mazor confirmed the breakaway on Monday. Israeli media said Netanyahu offered the defectors senior government posts.
Adding new lawmakers could reduce the likelihood of the government's dissolving over contentious issues including a court-ordered reform of the military draft and budgetary feuds.
Kadima quit the government last Tuesday just two months after joining, having quarreled with Netanyahu over draft reform.
The defection of the Kadima lawmakers would shore up Netanyahu's parliamentary majority to 72 of 120 seats.