Singh will be the second leader after Canadian Prime Minister Stephen Harper to boycott the Nov. 15-17 meeting. There are 54 members of the Commonwealth, a loose association of former British colonies.
Their decision is expected to sharpen focus on the demand by Western nations and rights activists that Sri Lanka account for thousands of civilians who are suspected to have died in the final months of a quarter-century civil war that that ended in 2009 when government forces crushed separatist Tamil rebels.