Police chiefs must avoid “near silence” in the wake of atrocities to prevent conspiracy theories spreading, the terror watchdog has revealed.

Jonathan Hall KC warned others will fill the vacuum if officers “do not take the lead in providing clear, accurate and sober details”.

False information spread on social media in the hours after the Southport killings led to riots, mass protests and public fury.

Mr Hall rejected Prime Minister Sir Keir Starmer’s calls for the terrorism definition to be widened to include mass killings, warning it could hit free speech and create a new “landscape” where thousands more people are labelled terrorist offenders.

He declared a new offence should be created to cover planning to carry out slaughters, with a maximum penalty of life behind bars.

Speaking about whether acts of extreme violence, where multiple people are killed, should be declared terrorism, Mr Hall said: “In unclear cases, especially if the attacker has been killed or makes no comment in police, police may be dependent on the outcome of searches of electronic devices.

“A clear and honest explanation to the public would go something like this: ‘Because of the nature of the attack, Counter Terrorism Police are involved in this investigation alongside the local force.

“Investigators are keeping an open mind, but there is insufficient evidence at the moment to state why the attack was carried out and whether terrorism was involved’.

“In the digital era, if the police do not take the lead in providing clear, accurate and sober details about an attack like Southport, others will.

“Social media is a source of news for many people and near-silence in the face of horrific events of major public interest is no longer an option.

“Following Southport, the disinformation generated on social media, combined with widespread allegations of a ‘cover-up’, risked far more prejudice to any trial than the placement of undisputed facts about the attacker in the public domain.”

The Independent Reviewer of Terrorism Legislation’s findings come as Prime Minister Sir Keir Starmer said Britain faces a new threat from “extreme violence carried out by loners, misfits, young men in their bedrooms” following the Southport murders.

Sir Keir said the law and framework for responding needed to be appropriate to the “new threat” and whatever legal changes were necessary would be made.

Axel Rudakubana was jailed for a minimum of 52 years for the murders of three girls and attempted murders of eight other children, who cannot be named for legal reasons, class instructor Leanne Lucas and businessman John Hayes at a Taylor Swift-themed dance class in July last year.

Despite contact with state agencies such as Prevent, aimed at countering terrorism, authorities failed to stop the attack which claimed the lives of Alice da Silva Aguiar, nine, Bebe King, six, and Elsie Dot Stancombe, seven.

Rudakubana’s acts of extreme violence were not considered terrorism under existing laws because there was no evidence of his purpose being to advance an ideological cause as set out in the terror definition.

Describing the need for a new offence, Mr Hall said: “I therefore recommend that the Government considers bringing forward legislation to create a new offence where an individual, with the intention of killing two or more persons, engages in any conduct in preparation for giving effect to this intention.

“The maximum sentence should be life imprisonment.”

Mr Hall said “Cold realism is needed in the face of any suggestion that it is possible to reverse-engineer from the atrocious online viewing of the Southport attacker to identify a point at which he could have been stopped from knowledge of his browsing history alone.

“Online rhetoric rarely reflects online intentions. Many young people view and share terrible images of violence and sexual harm online, including terrorism content and make dreadful boasts about their intentions, but only the tiniest fraction of these will take real world steps to violence.

“There is no supercomputer or algorithm that can magically scan all online communications and tell who is an attacker and who is a fantasist.”

“Just because a person reads something does not mean he is persuaded by the cause presented in the material.”

Widening the terrorism definition any further could lead to thousands more people being labelled terrorism – even those sharing violent war footage.

Mr Hall added: “Such is the functional importance of the terrorism definition, that redefinition would alter the landscape.

“It would risk major false positives – the prosecution of people who by no stretch of the imagination are terrorists – and extend terrorism liability into novel terrain.

“People swapping violent war footage would be at risk of encouraging terrorism, resulting in unacceptable restrictions on freedom of expression.”

He added that the current “very wide” definition depends on the discretion of the police and authorities on deciding who to arrest and prosecute.

“It has been my experience that this discretion has been exercised capably and well, so that individuals are not exposed to irrationality, heavy-handedness or bias.

“But this is a product of a mature and defined system operating with a familiar threshold. Altering the threshold would not only expand the reach of terrorism legislation but would increase the possibility of inaccurate use and, in theory, abuse.”

He said for some cases it will be clear to investigators the attack was for “purely personal motives” and saying it is not treated as terrorism makes “good sense to do so”.

He referenced the case of Nicholas Prosper, 19, who murdered his mother and two siblings and was on his was to carry out a mass shooting at his old primary school when he was stopped by police.

A loaded shotgun was found hidden in bushes nearby, with a bag of more than 30 cartridges, when officers spotted him in Bramingham Road, Luton.

There is not a specific offence that he could be charged with for planning the school shooting because his plot would not be defined as terrorism.

“The key point to make is that terrorism legislation is not the UK’s main protection against this sort of attacker: what counts is gun control,” he said.

He added of school shootings and young copycats that it is “foreseeable” that other types of violent attacks will start a copycat craze, “most likely amongst the cohort of isolated often bullied teenagers with poor mental health, neurodivergence or personality disorder for whom grudges and grievances become reasons for violence.

“Few will be terrorists applying the definition.”