Reiterating the UK's commitment to hold those responsible for mass atrocities in Sri Lanka accountable, the British prime minister, David Cameron reminded Sri Lanka ahead of his bilateral discussions with the country's new president, Maithripala Siresena this afternoon, that "in six months time the spotlight will be back on".

"Ever since my visit to Sri Lanka in 2013 one thing has remained constant - my unwavering commitment to stand up for all those affected by what happened. I remain determined to ensure that there is accountability for the past and respect for human rights today. And that will be my message to President Sirisena when I meet with him in Downing Street today," Mr Cameron said, writing in the Tamil Guardian.

Last month the UN Human Rights Council voted to defer the publication of the findings of a UN inquiry into mass atrocities committed against the Tamil people at the final stages of the armed conflict by six months to September, giving the new government in Sri Lanka six months to cooperate with the OHCHR Investigation on Sri Lanka (OISL).

The decision prompted widespread demonstrations by Tamils in the North-East, who said they were very disappointed that they are once again denied justice.

"I know this decision has not been welcomed by everyone who has an interest in Sri Lanka’s future. And, let me assure you, we did not do this lightly" wrote Mr Cameron, acknowledging the feeling of the Tamil people and some human rights groups.

"But we did it because we want to see genuine reconciliation in Sri Lanka. And we believe that the extra time enables the new Sri Lankan government to engage with the UN investigation, as it has committed to do, and to establish its own credible accountability processes."

"I will encourage President Sirisena to seize this window of opportunity."

Recent weeks have seen Tamils in the North-East rejecting a domestic process of accountability as lacking credibility however. Last month thousands joined a march by students and staff of the University of Jaffna rejecting an internal justice mechanism and asking how those who were in power at the time of the mass killings of the Tamil people, could be trusted to investigate themselves. The Tamil party led Northern Provincial Council also passed a resolution calling for an international investigation of what it said was the genocide of the Tamil people by successive Sri Lankan government.

Their sentiments were echoed on Monday in London, as British Tamils protested against Mr Sirisena's visit to the UK, calling him a "war criminal" for his role as acting defence minister of Sri Lanka during the final two weeks of the armed conflict when the majority of Tamil civilian deaths by government shelling took place.

Calling on the new government to rebuild trust with the Tamil people, Mr Cameron wrote: "The Sri Lankan government must keep up the pace on reform. Building trust by demilitarising the North, handing more land back from the military to local communities and releasing detainees held without charge."

The issue of disappearances continues to simmer across the North-East with mothers across the region launching protests demanding that the new government returns their missing children to them.

Recalling his visit to Jaffna in 2013, ahead of the Commonwealth leaders' summit, where he met Tamils whose loved ones were missing or killed, Mr Cameron said the victims will not be forgotten.

"I will never forget the faces of those I met in Jaffna over a year ago. Their stories of unbearable suffering and loss will stay with me forever and continue to drive me in pushing for change. What I saw and heard also underlined why I went there in the first place: to shine a light on the lack of progress and to help bring about international pressure for reform."
 
"We owe it to victims and survivors across Sri Lanka to ensure that those responsible are held to account. And we will not forget them. That’s why we, and the UN High Commissioner for Human Rights, have made clear that the report must be published and discussed by September 2015 and we will stick to that."
 
"In six months time the spotlight will be back on Sri Lanka. Let it shine on a nation that is prepared to address its past, reconcile its differences and reform its political structures so it can secure a bright future for all its peoples. And let it shine on a renewed and growing partnership between the UK and Sri Lanka."