Indian PM Manmohan Singh will boycott this week's Commonwealth summit in Colombo, officials say, amid a row over Sri Lanka's human rights record.
Mr Singh has written to Sri Lankan President Mahinda Rajapaksa to say he will be represented by his foreign minister instead, a spokesman says.
Sri Lanka put the decision down to "internal political reasons" in India.
Mr Singh is the second government head, after Canada's Stephen Harper, to boycott the summit.
UK Prime Minister David Cameron has said Mr Rajapaksa has "serious questions" to answer about alleged war crimes carried out in the final months of the campaign against Tamil Tiger rebels in 2009.
Mr Cameron said he would urge Mr Rajapaksa to hold an independent inquiry into allegations of indiscriminate shelling, extra-judicial executions and rape of Tamil civilians.
The Sri Lankan government has strongly denied the claims.
Sri Lankan Foreign Minister GL Pieris told the BBC on Sunday that Mr Singh's decision was "not a defeat for us".
"Had he come we would have been really happy. This decision is taken because of [India's] internal political reasons, but it will not affect the success of this programme, we don't think it is a problem."
Opposition Labour foreign affairs spokesman Douglas Alexander called on the UK prime minister urgently to consider reversing his decision to attend the summit.
But Foreign Secretary William Hague said the government had decided that staying away from the meeting would damage the Commonwealth.
The BBC's Sanjoy Majumder in Delhi says there has been growing pressure on Manmohan Singh to boycott the summit, especially from Tamil politicians in India.
However, critics of the decision said it would open the door for giant Asian rival China to extend its influence in the region. China already helps to fund Sri Lankan military and infrastructure projects.
"Now we are vacating our backyard for the Chinese to rebuild all of a booming post-war Sri Lanka," influential columnist and editor Shekhar Gupta wrote in the Indian Express newspaper.
The Commonwealth is an organisation of 53 former British territories.
In October, Mr Harper said he had not taken the decision to boycott the meeting lightly.
But he said a lack of accountability for human rights abuses "during and after the civil war is unacceptable".
Canada is home to the largest expatriate Sri-Lankan Tamil community. Many migrated over the past few decades as civil war gripped Sri Lanka.
In another development on Sunday, Australian Greens Senator Lee Rhiannon and New Zealand Greens MP Jan Logie were briefly detained by Sri Lankan immigration officials in Colombo.
Ms Rhiannon's office tweeted that they had been on a fact-finding mission, investigating human rights issues and press freedom.
She later told the Sydney Morning Herald they were accused of breaching visa laws and were held for three hours before being released.
Sri Lanka's army defeated the ethnic-Tamil rebels after a brutal 26-year war that left at least 100,000 people dead.
Both sides were accused of human rights abuses throughout the conflict, with much focus on its final stages when thousands of civilians were trapped in a thin strip of land in the north of Sri Lanka.
Estimates of civilian deaths in the final months range widely from 9,000 to 75,000.
The Sri Lankan government commissioned its own investigation into the war in 2011.
It cleared the military of claims that it deliberately attacked civilians. It said that there had been some violations by troops, although only at an individual level.