Cabinet Office: Intelligence: Review of the Intelligence on the London Terrorist Attacks on 7 July 2005 (published May 2009) "This Report addresses the unanswered questions relating to the intelligence on the 7/7 bombers, particularly those arising out of the investigation of the 2004 fertiliser bomb (CREVICE) plot." Linkpage to the pdf report [1,401KB, 108 pages] and related documentation.
Many perspectives on this. For example, see BBC News, MI5 'too stretched' before 7 July, 19 May 2009
Telegraph, July 7 report key points: 'astounding' scale of problem faced by MI5, 19 May 2009
Mirror, Terror chief admits MI5 did not have enough agents to target the 7/7 bombers, 20 May 09