Not only would it undermine credibility of UK Leave, but elements of the Conservative party.
The is the same report which the Conservative government took almost 18 months to publish after it was finished [1] (and which the government did what they could to avoid starting). This includes the period during which a Conservative government won a general election on the platform of the very issue which prompted the report in the first place - the Brexit referendum.
And when was it finally released? The day before parliament's summer recess - to minimise accountability and discussion. By the time parliament sits again it will be just 4 months out from a no deal Brexit, meaning that it has only been available for discussion for 8 weeks out of the 4 and a half years that it has had relevance (since the referendum).
And the same report which is due to be scrutinized by a non-partisan Intelligence and Security Committee. The same committee that the prime minster has installed his preferred puppet to chair (who incidentally has no experience in intelligence or security) [2], booting a senior minister who opposed the move out of the party in the process.
The same committee which was once led by a former senior Conservative MP, now independent, who has expressed his anger over there being "no valid reason" to not publish it sooner [3].
This is more than just a bit of spin covering up some incompetence - it's democratic integrity being brushed aside.
[1] https://www.bbc.co.uk/news/uk-politics-53480682
[2] https://www.bbc.co.uk/news/uk-politics-53422010
[3] https://www.bbc.co.uk/news/av/uk-politics-53488971/grieve-on...