British MPs will be directed on Thursday towards a series of YouTube videos which form a key part of the Government's dossier of intelligence justifying imminent missile strikes to punish Bashar al-Assad's regime for using chemical weapons.
The intelligence summary, compiled by the Joint Intelligence Committee (JIC), will be handed to MPs before today's vote and concludes that last week's gas attack on a Damascus suburb could only have been carried out by Syrian government forces.
The dossier will argue the rebels do not have artillery capable of delivering chemical weapons. MPs will also be pointed to publicly available information including YouTube videos of the attacks which sources described as "utterly compelling".
The document to be given to MPs before today's Commons vote on Syria will also point to new estimates that the number of dead in last week's attack could be up to 1,000.
A Whitehall source said: "The JIC conclusion is that this could not have been anyone other than Assad." The dossier will be released amid growing calls for Barrack Obama and David Cameron to substantiate assertions there is "no doubt" Assad's regime committed the attacks.
All sides accept that a chemical attack took place in Ghouta on Aug 21, but Syria and Russia say rebel forces were responsible. The White House and American intelligence chiefs are preparing their own dossier which is expected to include intercepted messages from Syrian defence officials just hours after the attack.
One official made panicked phone calls to the head of a chemical weapons unit and demanded to know what was going on, according to Foreign Policy, an international affairs website.
America also has Israeli intelligence reports showing the regime was moving chemical weapons stocks into the area before the attack, according to the Wall Street Journal.
The intercepted message does not conclusively establish who gave the order to use chemical weapons and could mean it was a mistake, analysts suggested.
One US official said it was "unclear where control lies. Is there just some sort of general blessing to use these things? Or are there explicit orders for each attack?"
Yezid Sayigh, of the Carnegie Middle East Centre, said that while Mr Assad would have established the broad policy of using chemical weapons it seems unlikely commanders would need specific permission for each attack. He said it was possible the attack last week was "a misjudgment".
Shashank Joshi, of the Royal United Services Institute, said: "What's driving the certainty is the intercept evidence, particularly of communications from Syrian defence officials to field units, in conjunction with prior evidence of stockpiles being moved into the area.
But this is not just about the Ghouta incident, this is about the accumulated evidence of all those attacks going back to the spring." Lakhdar Brahimi, the UN's special envoy to Syria, said he had yet to see the evidence gathered by the US, France and Britain, adding: "We will be very, very, very interested in hearing from them."