A committee of MPs at Westminster has concluded that a lack of resources prevented the security services from intercepting the 7 July 2005 London bombers.
However, the report - by the Intelligence and Security Committee - says there was no evidence of an intelligence failure in the run up to the attacks.
Commenting on a second report into the attacks by the Home Office, which was also published today, the British Home Secretary, John Reid, told MPs that there was 'no evidence' to suggest that there was a fifth bomber involved in the attacks.
52 people, as well as the four bombers, died in the attacks on three underground trains and a London bus.
The Intelligence and Security Committee is chaired by former Northern Secretary Paul Murphy.
Its report said that two of the bombers had been known to security officers but the threat they posed was not realised.
The ringleader, Mohammed Sidique Khan, had been under surveillance but MI5 officers assigned to him were diverted to another operation.
Today's report concludes that the chances of preventing the attacks might have been greater had different investigative decisions been made by the security services.
However, referring to the ongoing terrorist threat to Britain, the MPs warn that even if the work of the security services were to be more intrusive, it is highly unlikely it would be able to stop all terrorist attacks in the future.
The report is unlikely to satisfy victims of the bombings, and their relatives, who have been calling for a full public inquiry into the events leading up to the attacks.