The families of those killed in the London Bridge terrorist attack have asked a judge to find that “unreasonable failures” by MI5 and police may have led to chances to stop the atrocity being missed.

Eight people were killed in the attack in June 2017 after three terrorists first ran over pedestrians in a van then went on a stabbing rampage.

Inquests into the deaths end this week and formal submissions to the coroner before he makes his findings were submitted on Wednesday morning.

The submission from six of the families of those who were killed asks the chief coroner for England and Wales, Mark Lucraft QC, to directly criticise counter-terrorism investigators.

The security service had the attack ringleader Khuram Butt under investigation at the time of the atrocity, having started an investigation into him in 2015, supported by the Metropolitan police’s counter-terrorism command.

The six families say in their submission: “Accordingly, it is submitted that the narrative conclusions in each case should refer to the possibility that the attack could have been prevented had it not been for unreasonable failures in the investigation into Khuram Butt.”

The six families, through their barrister Gareth Patterson QC, say preparations for the plot over several months were missed: “Evidence of attack planning can probably be dated to the meetings of 7 March 2017 and the purchase of the operational telephone on around 15–17 March 2017.

“This means that there was a period of months during which an attack could have been detected.

“It is submitted that there is a ‘substantial chance’ that a higher level of monitoring would have detected such planning. It is important that the paucity of actual evidence of attack planning, in the context of a failure to investigate, is not used to suggest that there was no evidence that might have been uncovered through proportionate investigative steps.”

Official inquiries cleared both police and MI5 of wrongdoing. MI5 has the lead for identifying terrorist suspects and attack plots, and vehemently denies the attack was preventable.

The six families are those of Christine Archibald, who was killed by the van, as well as Sara Zelenak, James McMullan, Alexandre Pigeard, Sébastien Bélanger and Kirsty Boden, who were all stabbed to death.

Rebutting the allegations, government lawyers acting for MI5 reject any claims that the security service’s actions meant the state’s responsibility to protect life was breached. That obligation is contained in article 2 of the European convention on human rights, to which the UK is a signatory.

The government lawyers say: “The evidence before the coroner does not reveal an arguable, or prima facie, case that there was a failure on the part of MI5 to take reasonable steps to avoid a real and immediate threat to the lives of the victims of the attack, of which they were, or should have been, aware at the time. Accordingly, article 2 is not engaged in respect of the involvement of MI5 in this case.

“This was an attack planned in secret, by the three attackers alone. The exhaustive post-attack investigation has not revealed a single piece of intelligence which, had it been identified at the time, would have alerted MI5 to an imminent attack. The attackers appear to have kept their plan secret from even those closest to them. The only route by which it is possible to draw a link between an additional investigative step and the attack plan is … to engage in speculation.

“There is no proper basis for a conclusion that MI5 failed to take reasonable steps in the face of a real and immediate risk.”

British authorities also potentially face criticism over the failure to have protective barriers on London Bridge, which may have stopped two victims being run over and killed by the van. The six families, in their legal submission, say: “It is arguable that there was a breach of the state’s operational duty to protect life in respect of protective security on London Bridge. The families agree that there was no adequate system.”

The London Bridge attack came after a terrorist killed pedestrians three months earlier, in March 2017, in a vehicle attack on Westminster Bridge.

The chief coroner will sum up and deliver his findings on Thursday and Friday.