The barrister David Anderson, who conducted the inquiry into the four terrorist attacks in the UK this year, chooses his words carefully – his tendency is towards understatement rather than anything excitable.

So when Anderson concluded in his official report that the Manchester Arena suicide attack could have been prevented, that is a serious criticism of MI5.

It is the most damning finding against the UK’s domestic intelligence agency since inquiries by the parliamentary intelligence and security committee (ISC) after the murder of Lee Rigby in 2013 and the 2005 London attacks.

Comparisons with 2013 and 2005 are striking. The Manchester suicide attacker, Salman Abedi, was known to MI5 just as those in 2005 and 2013 were known to the agency.

MI5 failed to appreciate the value of intelligence on Abedi on two occasions just months before the attack, just as clues were missed in 2005 and 2013.

There is no elaboration in the Anderson report on what is meant by intelligence received about the Manchester attack not being “fully appreciated”. Was the information so scanty as to make a considered assessment difficult? Or was the data misunderstood? Those seeking answers may have to wait for any court case.

There is an echo in the Anderson report of the ISC findings in the Rigby case that while the errors would not individually have affected the outcome, cumulatively they might have made a difference. He cannot say with any confidence the Manchester attack could have been stopped, but it might have been.

The comparison between the Anderson report and the one on the 2005 London attacks is even more striking. Although the ISC cleared MI5 for failing to keep the attackers under surveillance, it tellingly criticised a lack of intelligence sharing between MI5 headquarters in London and police special branch in West Yorkshire, where the attack was organised.

Anderson does not point to any specific failure to share intelligence in the four attacks this year but he notes one of the recommendations is to share intelligence-derived information more widely, including through neighbourhood policing.

Failure to share intelligence is a frequent failing on the part of intelligence agencies worldwide, stemming from a reluctance to disseminate hard-won information and a fear of compromising sources.

MI5 knows it is always going to get a kicking after any inquiry. It would probably accept the criticism of its handling of this year’s attacks is comparable with 2013 and 2005.

The agency argues, too, it is easy with hindsight to spot mistakes but that there are thousands of potential terrorists and not enough staff to mount 24-hour surveillance.

There is another argument, stemming from alarming changes the intelligence community has identified this year.

A senior intelligence officer recently spoke nostalgically of intelligence gathering a decade or so ago as a golden age. There was less terrorist activity. Plotting was usually over a long period, making it easier to pick up. Interception of communications and monitoring was relatively easy, given the lack of encryption.

He made the contrast with this year. What worried him most was the speed of radicalisation: a potential terrorist recruit could within days see propaganda online, be diverted to a site on how to build a bomb and then mount an attack.

The response of the government and the intelligence agencies has been to make counter-terrorism a priority. At least one counter-terrorist exercise or drill is held daily in the UK, with staff and resources having been diverted from other intelligence targets.

Islamic State has more or less been defeated in Iraq and Syria. The intelligence assessment is that those Britons who survived are not heading back to the UK but to other countries in the Middle East to bombard the UK with propaganda online. This is the online front on which intelligence community is focused.

The intelligence officer says, as does everyone else in the intelligence community, that there is no way to stop every terrorist plot. “There is no silver bullet,” he said.