David Cameron vowed to send a tough message to Sri Lankan President Mahinda Rajapaksa over allegations of "chilling and appalling" war crimes upon arriving in Colombo for the Commonwealth summit.
The prime minister, who defended his decision not to boycott the summit, said: "You can't make the arguments unless you are there."
The prime ministers of India, Canada and Mauritius have decided not to attend the meeting in protest at perceived war crimes in Sri Lanka.
The biennial summit known as Commonwealth Heads of Government Meeting (Chogm) is taking place from 15 to 17 November in Colombo. It is being chaired by Prince Charles.
The Commonwealth is a loosely-formed association of 53 former British territories.
The British premier told reporters in Colombo: "There are some important points to put to the Sri Lankans. There is the problem of human rights as we speak today: the people who have disappeared; the lack of free rights for journalists and a free press.
"But I think perhaps most important of all is the need for proper investigations to look into what happened at the end of this very long, appalling civil war."
The 26-year-long war ended in 2009, and in the last five months of the bloody conflict alone, as many as 40,000 civilians were estimated to have been killed. The Sri Lankan administration has denied any hand in the killings.
Cameron is set to hold talks with Rajapaksa over alleged violence against Tamils and other groups on the South Asian island. There are also concerns that the encounter may spark a diplomatic row between Sri Lanka and the UK.
On the eve of the summit, Cameron was in New Delhi holding talks with his Indian counterpart Manmohan Singh. The Indian prime minister was forced to pull back from participating in the summit following intense pressure from Tamil political outfits and leaders in his country which faces an election by the middle of next year.
Cameron said in Delhi: "India, Canada, Britain... we all have the same approach to Sri Lanka, which is that we want to see greater efforts of reconciliation, we want to see better efforts on human rights, we want to see proper inquiries into what happened at the end of that dreadful civil war. So, there is a lot of difference in the policy."
"Of course, there is always a case for not going somewhere. But, I think, actually, we will get further by going and having conversations with the Sri Lankans about what needs to happen and shining a light on some of the issues and the problems that are there. And because I am going to the north of the country, I will be taking journalists with me."