No 10 publishes assessment as MPs are put on notice they may have to return to Commons for weekend vote on military action

Britain's joint intelligence committee (JIC) has concluded it is "highly likely" that the regime of Bashar al-Assad was responsible for the chemical weapons attacks in Syria last week that have prompted moves towards launching military strikes.

In an echo of the buildup to the Iraq war in 2003, Downing Street took the rare step of releasing the assessment of the JIC to support its case that the Assad regime was responsible.

But the assessment was mainly based on "open source" evidence such as video footage of the victims and a judgment that the opposition does not have the capability to launch such an attack.

Downing Street sources said the intelligence, outlined by David Cameron to Tory MPs at a Westminster meeting, presented a "compelling and conclusive" case of the involvement of the Assad regime in the chemical attack.

MPs, who are due to hold an initial non-binding vote on Thursday night, are being put on notice that they may have to return to Westminster on Saturday or Sunday to authorise military action in a second vote. Tentative plans for the first weekend sitting of parliament since the Falklands war in 1982 are being made amid signs that the White House is willing to show Britain some flexibility but cannot wait indefinitely.

One source said No 10 was now confident Cameron would win the vote on Thursday night after setting out the legal case and intelligence evidence to Conservative MPs during a pre-debate meeting. The source said MPs spoke out "overwhelming" in support of the government's motion approving "legal, proportionate" military action in Syria if necessary. "We're confident the argument is being made," the source said.

The confidence in No 10 came as a report said the US evidence linking the Assad regime to the attack was no "slam dunk", and there remains uncertainty over real control over Syria's chemical weapons stockpiles.

The Associated Press quoted "multiple US officials" as pointing towards gaps in the US intelligence picture, in strong contrast to the certainty expressed by American leaders, including Barack Obama. The AP said the US intelligence evidence against Syria was "thick with caveats".

The questions over intelligence came as the British government tried to build support by releasing the JIC assessment and a statement based on the formal legal advice by the attorney general, Dominic Grieve, that limited military strikes to deter future chemical weapons attacks would be in line with international law.

In its assessment of the chemical weapons attack the JIC said the regime had launched at least 14 chemical weapons attacks in the past. The assessment says: "It is not possible for the opposition to have carried out a CW attack on this scale. The regime has used CW on a smaller scale on at least 14 occasions in the past. There is some intelligence to suggest regime culpability in this attack. These factors make it highly likely that the Syrian regime was responsible."

The JIC acknowledged that some of its assessment was based on "open source" evidence such as testimony from victims, doctors and video footage. But in a separate letter to Cameron the JIC chairman, Jon Day, said he had seen "highly sensitive" unpublished intelligence that supported their view that the regime had launched the attacks to clear the opposition from strategic parts of Damascus.

The AP report says of the US intelligence: "It builds a case that Assad's forces are most likely responsible while outlining gaps in the US intelligence picture. US officials have publicly said that Washington's certainty about the regime's responsibility for the chemical attacks was based on its conviction that only the government had access to chemical weapons."

The JIC assessment was released at the same time as a government statement on the legal advice from the attorney general. This said that military action would be legal "under the doctrine of humanitarian intervention". It also said action would be legal even without the authority of the UN security council.

The statement said: "If action in the security council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria by deterring and disrupting the further use of chemical weapons by the Syrian regime. Such a legal basis is available, under the doctrine of humanitarian intervention."

The Downing Street source said YouTube videos showed gas had been deployed against Syrian citizens. No 10's argument is that the use of chemical weapons has been overseen by the regime, whether or not the attacks were ordered by Assador "rogue elements", the Downing Street source said.

There is palpable anger in No 10 about Ed Miliband's last-minute decision to oppose the government. A source said Cameron was determined to vote against Labour's motion because it "doesn't even condemn Assad". He also criticised Labour for raising a "red herring" after the opposition insisted Britain must wait for a report from UN weapons inspectors before acting against Syria.

A No 10 spokesperson said: "The judgment of the joint intelligence committee is that a chemical weapons attack did occur in Damascus last week; that it is highly likely that the Syrian regime was responsible; that there is some intelligence to suggest regime culpability; and that no opposition group has the capability to conduct a chemical weapons attack on this scale.

"The government's position on the legality of any action makes clear that if action in the UN security council is blocked, the UK would still be permitted, under the doctrine of humanitarian intervention, to take exceptional measures including targeted military intervention in order to alleviate the overwhelming humanitarian suffering in Syria.

"The cabinet endorsed the recommendation from the national security council and the prime minister will set out the proposed government response in the house this afternoon.

"Ministers agreed that it is fundamentally in our national interest to uphold the longstanding convention on chemical weapons and to make clear that they cannot be used with impunity.

"Any response should be legal, proportionate and specifically in response to this attack and everyone around the cabinet table agreed that it is not about taking sides in the Syrian conflict nor about trying to determine the outcome."