Canadian prime minister Stephen Harper has already said he would boycott the Commonwealth Heads of Government Meeting that Sri Lanka is hosting from Friday to Sunday.
Mr Harper said last month he was disturbed by continuing reports of intimidation and incarceration of political leaders and journalists, the harassment of minorities, reported disappearances and accusations of extra-judicial killings.
The Sri Lankan government, which defeated the separatist Tamil Tiger rebels in 2009, faces increasing pressure from the international community to try those responsible for rights abuses during a nearly three-decade-long civil war.
But prime minister David Cameron has insisted he will attend. Yesterday, foreign secretary William Hague insisted Britain would have “more impact” by raising concerns in the country.
The Scottish Government’s Commonwealth minister Shona Robison is also travelling to the country this week to promote the Commonwealth Games in Glasgow next year.
Mr Singh has now written to Sri Lankan president Mahinda Rajapaksa to say he will be unable to attend and foreign minister Salman Khurshid will take his place instead, an Indian foreign ministry spokesman said.
“From time to time the prime minister is required to be here and he is unable to visit,” Mr Khurshid told reporters.
At home, Mr Singh’s move is also being seen as bowing to pressure from India’s own large Tamil population, with an eye to a general election that must be held by next May.
Mr Cameron has rejected calls from Labour, rights activists and Tamil groups to join the boycott.
Instead, he has pledged to put “serious questions” to Mr Rajapaksa about his regime’s widely-condemned human rights record and make a rare visit to minority Tamil areas in the north of the island.
Asked about the Indian move, Mr Hague said: “We have decided that if we were to stay away...it would damage the Commonwealth without changing these positively in Sri Lanka.”
“Sri Lanka is in the spotlight so let’s make full use of it being in the spotlight. Rather than sit in London and talk about it, we will be there. It will make more impact in Sri Lanka with the Prime Minister and me there doing that, than sitting in our offices in London.”
Downing Street said it recognised attendance posed a “difficult” choice for some countries but that it was up to each to determine its own approach.
UN high commissioner for human rights Navi Pillay has warned that Sri Lanka – considered a “country of concern” by the Foreign Office – is heading in an “increasingly authoritarian direction”.
The Sri Lankan government denies allegations that it has been complicit in kidnappings, torture and other abuses.
Mr Cameron issued a fresh call for action after a “chilling” documentary. The No Fire Zone, commissioned by Channel 4 which has reported extensively on the issue, features footage of apparent war crimes.