Cameron made his stance clear after visiting the former war torn Northern part of Sri Lanka Friday and having a meeting with Sri Lanka President Mahinda Rajapaksa, media reported. (Agencies)
Cameron is the first foreign leader to visit Jaffna since Sri Lanka's independence in 1948 and he insisted that the visit "gave a voice to the Tamil people in the north and it is a voice the world should listen to".
"I have made it clear to Sri Lanka's president that he now has a real opportunity to show magnanimity and reform and build a successful and inclusive future for his country and I very much hope he seizes it," he told reporters.
"Let me be very clear. If an investigation is not completed by March then I will use our position on the UN Human Rights Council (UNHRC) to work with the UN Human Rights Commissioner and call for a full, credible and international inquiry."
Cameron has come under intense pressure from critics of the Sri Lankan government to boycott the ongoing Commonwealth Heads of Government Meeting (CHOGM) in Colombo.
Canada led the way in absenting itself from this important meeting of the 53-member Commonwealth bloc with India and Mauritius Prime Ministers also skipping the summit.
In fact Mauritius by its boycott gave up the chance to host the 2015 CHOGM.
However, Cameron took a balanced approach, insisting that Sri Lanka is a country with "enormous potential" that has achieved impressive economic development in a short period of time.
Nonetheless, human rights excesses cannot be ignored, he said and outlined steps that Britain expects the Sri Lanka government to make before March 2014 when it returns before the UNHRC.
"That means working in partnership with the newly elected chief minister of the Northern Province, it means freedom for the media and it means transparent, credible and independent investigations into alleged war crimes," Cameron said at a press conference.
Cameron made his stance clear after visiting the former war torn Northern part of Sri Lanka Friday and having a meeting with Sri Lanka President Mahinda Rajapaksa, media reported.