David Cameron is pushing for an independent, international investigation into alleged atrocities during the Sri Lankan civil war to begin within a year as pressure mounts on the regime in Colombo.
The Prime Minister has personally sought the support of several fellow leaders for a United Nations Human Rights Council (UNHRC) resolution authorising a UN-led probe.
As many as 40,000 civilians are estimated to have died in the final months of the Sri Lankan government's 26-year fight with Tamil Tiger separatists - which ended in 2009.
Both sides in the brutal conflict are accused of widespread human rights abuses - including executions and rapes of prisoners by state troops.
On a visit to the island in November for a Commonwealth summit, Mr Cameron warned president Mahinda Rajapaksa that he had until this month to set up a credible domestic inquiry.
A probe into some 16,000 people still listed as missing has been dismissed by critics as a sham and there has been little or no progress on promises to set up a truth and reconciliation commission.
Mr Cameron confronted the president after meeting displaced Tamils who have spent more than 20 years in a temporary camp, during the first visit in 60 years by a world leader to the battle-scarred north.
An angry Mr Rajapaksa, who denies claims of war crimes and ongoing abuses against the minority Tamils, accused Mr Cameron of seeking votes from the UK's large Tamil community.
The UN High Commissioner for Human Rights, Navi Pillay, has concluded Colombo does not have the will to investigate properly and will present her latest assessment at this month's UNHRC meeting.
The UK - with the US, Montenegro, Macedonia and Mauritius - has tabled a resolution due to be voted on at the Geneva gathering which includes provision for her office to lead the detailed investigations.
British Tamils have criticised the "insubstantial" draft for renewing previous calls on Colombo to conduct its own probe and requiring another assessment by the Commssioner.
But Downing Street made clear Mr Cameron believed Mr Rajapaksa had "failed" to meet his demands and was keen for an international probe to be up and running within 12 months.
"When the Prime Minister was in Sri Lanka, he said that this month was the deadline for the Government of Sri Lanka to take concrete action to establish a credible, thorough inquiry into what happened during their civil war, including its final stages," a Number 10 spokeswoman said.
"Therefore the UK, along with four other countries including the US, has tabled a resolution on Sri Lanka at the UN Human Rights Council supporting the call by the UN High Commissioner for Human Rights for an international, independent investigation into violations of human rights and related crimes by both sides during the war.
"We want this to take place in this year's UNHRC session. There will be a vote on our resolution at the end of this month."
The UK was " working hard to secure support from other countries", she said, with the PM writing to the leaders of several countries represented on the Council - Mexico , Sierra Leone and South Korea.
It came as Channel 4 disclosed what it said was m obile phone footage of soldiers celebrating the deaths of Tiger fighters and performing "acts of grotesque sexual violation" on the bodies.
The Sri Lankan High Commission dismissed the film - which the broadcaster said had been verified as genuine by an independent expert - as "unmitigated and unsubstantiated rubbish".