Singh sent a letter to Sri Lankan President Mahinda Rajapakse on Sunday telling him of his decision not to attend the Commonwealth Heads of Government Meeting (CHOGM), which has become mirred in controversy over demands for Colombo to address the allegations.
The prime minister was "unable to attend personally" the 53-nation summit which Sri Lanka is hosting from November 15-17, foreign ministry spokesman Syed Akbaruddin told AFP.
Singh's move is seen as bowing to pressure from India's own large population of ethnic Tamils to stay away in protest at the alleged massacre of Tamil civilians by Sri Lankan forces in the final months of the Tamil separatist war in 2009.
Several ministers from within Singh's government had urged him to stay away from the event, amid concerns about upsetting Tamil voters -- an important constituency -- months before India holds national elections.
Singh is sending his top foreign official, External Affairs Minister Salman Khurshid, to head the Indian delegati