The death toll among refugees trapped in the last Tiger redoubt in north eastern Sri Lanka is three times higher than that acknowledged by the government.
Sri Lankan authorities say their military observed a "no-fire-zone" and stopped using heavy weapons on April 27 when 100,000 Tamil civilians were trapped.
However aerial pictures and witness testimony suggest the army in fact launched a three week artillery barrage from the end of April.
Confidential United Nations documents say 7,000 civilians had died in the zone by the end of April and sources told The Times newspaper the toll then grew to around 1,000 a day.
The figures would put the final death toll, before the zone was finally overrun and Velupillai Prabhakaran, leader of the Liberation Tigers of Tamil Eelam (LTTE), was killed, at more than 20,000.
"Higher," a UN source told the newspaper, "Keep going."
Aerial photographs show makeshift sandy burial mounds in the no fire zone.
Navi Pillay, UN High Commissioner of Human Rights, has said the LTTE recruited child soldiers and used civilians as human shields during the conflict, while the military had indiscriminately shelled areas packed with civilians. Both sides have denied the allegations.
Hundreds of thousands of Tamil refugees are now being held in camps in Sri Lanka.
The UN estimates between 80,000 and 100,000 people died in the 26-year LTTE struggle for a separate state for Sri Lanka's minority Tamils.
A spokesman for the Sri Lankan High Commission in London rejected the newspapers allegations.
He said: "Civilians have not been killed by government shelling at all. If civilians have been killed, then that is because of the actions of the LTTE [rebels] who were shooting and killing people when they tried to escape."