A warning that this article contains graphic images.
Tens of thousands of photos that are said to document the execution and torture of prisoners of the Syrian regime are just the "tip of the iceberg", according to a leading war crimes prosecutor.
The images - 55,000 of them in all - were reportedly smuggled out of the country by a defector who says he took them himself.
One of three prosecutors to study the photos, Sir Desmond de Silva QC, told ITV News that they probably represent only a small proportion of the abuses.
Their report comes as world leaders meet in Switzerland to attempt to break the political deadlock and violence in Syria.
ITV News correspondent Dan Rivers reports:
The images came to light after a former police photographer defected - code-named 'Caesar' in the report to protect his identity - and handed them over to the Syrian opposition.
The Qatari government, which supports some rebel groups in Syria, commissioned a British law firm to investigate their validity.
The resulting report says that the photos document the killings of about 11,000 prisoners at one site in Syria.
Many of the corpses are emaciated and show signs of torture including strangulation, electrocution and beatings.
Foreign Secretary William Hague said the report "offers further evidence of the systematic violence and brutality being visited upon the people of Syria by the Assad regime".
"I've seen a lot of this evidence, it is compelling and horrific and it is important those who have perpetrated these crimes are one day held to account," he added.
A Syrian government source told ITV News that there was no proof the photos had been taken in Syria and dismissed the report as an attempt to attack the regime.
– Syrian government source
There is no proof about these photos. There is no proof these photos were even taken in Syria ... The report is from the government of Qatar who are not neutral ... They send money and weapons to the terrorists who are fighting in this country ... Look at the timing, this is designed to damage the talks, to attack the government of Syria and undermine the peace talks.
A spokeswoman for the US State Department said the images were "extremely disturbing" and that they "illustrate apparent actions that would be serious international crimes".
The report will pile pressure onto delegates attending a major summit in Switzerland aimed at finding a political solution to the conflict in Syria.
The talks - the first to be attended by representatives of both the Syrian government and opposition - are due to start on Wednesday.