Image caption UN inspectors are due to present their findings to the secretary general on Saturday

UK intelligence chiefs have told Prime Minister David Cameron it is "highly likely" the Syrian government was responsible for a chemical attack on 21 August, which killed at least 350 civilians in eastern Damascus.

The assessment was written by the Joint Intelligence Committee (JIC) on 27 August and released by Downing Street on 29 August.

The BBC's security correspondent Gordon Corera gives his analysis of the assessment below (GC).

Paragraph one

A chemical attack occurred in Damascus on the morning of 21 August, resulting in at least 350 fatalities. It is not possible for the opposition to have carried out a CW attack on this scale.

GC: A central reason for the relative confidence of the assessment is a view that it could not be the opposition and therefore had to be the regime which launched the attack.

The regime has used CW on a smaller scale on at least 14 occasions in the past.

GC: The accompanying letter from the chair of the JIC says that it has judged with the "highest possible level of certainty" that chemical weapons have already been used 14 times but not on the same scale. The JIC appears very confident that these attacks were by the regime and may have more intelligence about these incidents than the 21 August attack.

There is some intelligence to suggest regime culpability in this attack.

GC: This key sentence indicates that they have only "some" intelligence pointing to the regime carrying out the attack but nothing so conclusive as to dispel all doubt. It is described in the accompanying letter as a "limited but growing body of intelligence". It is also described as highly sensitive, meaning it might be intercepted communications or material from another country. The prime minister has been shown it, but it is not included in this assessment.

These factors make it highly likely that the Syrian regime was responsible.

GC: This is the key judgement of the document. The phrase "highly likely" indicates a significant degree of confidence but not absolute certainty.

Paragraph two

Extensive video footage attributed to the attack in eastern Damascus (which we assess would be very difficult to falsify) is consistent with the use of a nerve agent, such as sarin, and is not consistent with the use of blister or riot control agents.

GC: This paragraph, along with the accompanying letter, shows that the judgement that chemical weapons were used is based on what is known as open source information - in other words not secret intelligence but in this case public video footage. It also suggests they do not have separate confirmation of the use of chemical weapons, for instance in the form of analysis of samples at UK labs, which does seem to have taken place in the wake of other previous attacks. The committee chair says it has asked experts inside and outside government to see if this video could have been faked in any way by the opposition and has come to the conclusion that it is real.

Paragraph three

There is no obvious political or military trigger for regime use of CW on an apparently larger scale now, particularly given the current presence in Syria of the UN investigation team. Permission to authorise CW has probably been delegated by President Assad to senior regime commanders, such as [*], but any deliberate change in the scale and nature of use would require his authorisation.

GC: This judgement is important because it suggests the JIC have some idea of the chain of command for the use of chemical weapons but are still not sure why chemical weapons were used and on precisely whose orders on this occasion. The accompanying letter says this area of motivation is the one where it does not have high confidence in its assessment. There has been speculation as to whether the attack was launched on orders from the top or on the initiative of a local commander.

Paragraph four

There is no credible evidence that any opposition group has used CW. A number continue to seek a CW capability, but none currently has the capability to conduct a CW attack on this scale.

GC: This judgement is interesting because it tells us that some rebel groups have been trying to get hold of chemical weapons. There has been great concern that those opponents of the regime linked to al-Qaeda might get hold of them. However, the UK appears convinced that no opposition group would be able to carry out the kind of attack seen on 21 August, therefore meaning the use of weapons logically would have to have been by the regime.

Paragraph five

Russia claims to have a 'good degree of confidence' that the attack was an 'opposition provocation' but has announced that they support an investigation into the incident. We expect them to maintain this line. The Syrian regime has now announced that it will allow access to the sites by UN inspectors.

GC: The inspectors will be looking to prove if chemical weapons were used but are not expected to say by whom.

Paragraph six

There is no immediate time limit over which environmental or physiological samples would have degraded beyond usefulness. However, the longer it takes inspectors to gain access to the affected sites, the more difficult it will be to establish the chain of evidence beyond a reasonable doubt.

GC: A chain of evidence is required to be sure that a particular sample was not tampered with before it is analysed for proof that chemical weapons were used.