British experts at the Porton Down chemical warfare testing establishment have been involved in the assessment of samples taken from two sites in Syria, the Prime Minister said.
"We believe that the scale of use is sanctioned and ordered by the Assad regime," he said.
"We haven't seen any credible reporting of chemical weapons use by the Syrian opposition."
US President Barack Obama has tip-toed over the 'red-line' and armed Syrian rebels but it is a gamble of huge proportions.
Writing for ITV News, international affairs analyst David Butter looks at the timing of US support for Syria and when it may arrive.
Are the US supplied assault rifles too little, too late and what incentive is there for Assad to enter peace talks?