Witness said he was tasked to record deaths in custody from 2011 to 2013
There are 55,000 photos which lawyers say are evidence of extreme torture
Report made by ex-war crimes prosecutors who deem witness 'credible'
One lawyer said abuses are likely to be even more extensive
Foreign Secretary said the images are 'compelling and horrific'
Dossier commissioned by British lawyers for Qatar which supports rebels
Assad regimes questioned the authenticity of the photographs
There are 55,000 photos which lawyers say are evidence of extreme torture
Report made by ex-war crimes prosecutors who deem witness 'credible'
One lawyer said abuses are likely to be even more extensive
Foreign Secretary said the images are 'compelling and horrific'
Dossier commissioned by British lawyers for Qatar which supports rebels
Assad regimes questioned the authenticity of the photographs