The Prime Minister says elements within al-Qaeda have tried to obtain chemical weapons for use in Syria.
David Cameron says the Syrian regime probably approved use of chemical weapons on 19th March and 13th April.
He says he chose his words carefully on intelligence about al-Qaeda and chemical weapons, given what had happened in the past.
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?