NEW DELHI: Indian Prime Minister Manmohan Singh will skip this week's Commonwealth summit in Sri Lanka, the foreign ministry said Sunday, as pressure grows to boycott the event over alleged war crimes by Colombo.

Singh sent a letter to Sri Lankan President Mahinda Rajapakse on Sunday informing 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 skip 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 delegation.

Canadian Prime Minister Stephen Harper has already announced he would boycott the summit, to protest at Sri Lanka's failure to investigate its troops over allegations they killed up to 40,000 civilians in 2009.

British Prime Minister David Cameron has said he will attend the event, but has pledged to push for an international investigation into the allegations of war crimes.

Cameron's office also said in a tweet late Saturday that he will put “serious questions” to Rajapakse, after watching a “chilling documentary” about the events of 2009 that shows footage of alleged war crimes.

Sri Lanka responded Sunday by repeating its denial that its troops committed any such abuses.

“If a foreign head of government raises any issue with us, we will respond appropriately,” Rajapakse's spokesman Mohan Samaranayake told AFP.

“We deny that there were war crimes. What we tell them is, please provide the evidence and we will look into it.”

United Nations rights chief Navi Pillay last month warned Sri Lanka to show clear progress towards reining in rights abuses and investigating the suspected war crimes by next March, or face an international probe.

India has 62 million Tamils in its southern state of Tamil Nadu. They share close religious and cultural ties with their Sri Lankan counterparts.

The ruling Congress party is keen not to alienate potential supporters with elections due by May 2014, even at the expense of friction with its southern Sri Lankan neighbour.

Singh's move comes amid concerns about a crackdown in Sri Lanka against journalists, rights activists and others before CHOGM.

Sri Lankan immigration authorities on Sunday briefly detained an Australian and a New Zealand politician who were making a fact-finding mission into alleged human rights abuses.

Immigration officials held Australian Senator Lee Rhiannon and New Zealand MP Jan Logie at their hotel shortly before they were to hold a press conference about their mission, an opposition lawmaker said.

The pair were questioned about alleged visa violations before they were allowed to leave and fly home, officials said.

The move came less than two weeks after Sri Lanka kicked out two Australian media rights activists who were meeting local rights activists.

CHOGM is held every two years. Britain's Prince Charles will represent his mother Queen Elizabeth II, who is head of the bloc of mainly former British colonies.