A new offence to cover lone individuals planning non-terrorist mass killings should be considered in the wake of the Southport attacks, the UK’s terror watchdog has said.

However the independent reviewer of terrorism legislation, Jonathan Hall KC, says the definition of terrorism should not be expanded or changed.

Last summer, Axel Rudakubana stabbed six-year-old Bebe King, seven-year-old Elsie Stancombe and nine-year-old Alice Aguiar to death at a Taylor Swift-themed holiday class.

He also tried to murder eight other young girls, and two adults who tried to save them, including the class organiser Leanne Lucas.

Police officers discovered ricin pulp and a copy of an al-Qaeda manual at Rudakubana’s house, but did not find any clear ideological reason for his attack, so did not classify it as terrorism.

Other evidence suggested he was interested in Adolf Hitler, Genghis Khan, the Rwandan genocide and school massacres.

Following the attacks, Sir Keir Starmer said Britain faced a new threat from “extreme violence carried out by loners, misfits, young men in their bedrooms”.

In January he asked Mr Hall to identify any changes needed to the law as a result of the Southport murders.

Mr Hall considered whether the definition of terrorism itself should be changed to include mass violence without a political, religious, racial or other ideological motivation, but concluded it should not.

“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,” Mr Hall said.

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

However, Mr Hall said there was a clear risk to the public of people who are interested in carrying out acts of mass violence, including school massacres.

Currently there is no law against preparing for such an attack which “means that no prosecution would be available if the police raided an address and found careful handwritten but uncommunicated plans for carrying out a massacre”.

As a result, he is recommending the government considers creating a new offence, similar to the offence of preparing an act of terrorism.

The offence would make it illegal to “engage in any conduct” in preparation of the killing of two or more people, what he calls “non-terrorist mass casualty attack-planning”.

He suggests that the maximum sentence should be life imprisonment.

A government spokesperson said legislation would now be amended to incorporate Mr Hall’s recommendations, adding today’s report was “an important step” towards preventing an attack like this from happening again.

Downing Street also said police were looking into how misinformation spreads online after false information about the killings sparked riots across England and Northern Ireland.

The spokesperson added: “We have asked the Law Commission to conclude its own review into the rules around contempt of court as soon as possible.”

As part of his report Jonathan Hall KC considered what should happen with young people like Axel Rudakubana, who are referred to the counter-terrorism Prevent programme, but are not taken on because they do not have a clear ideology.

“If they are not to be managed by counter-terrorism police, who will ‘own’ the risk?”, he asked.

He said lessons should be learned from counter-terrorism, and generally it should be the police that take responsibility for public protection, rather than leaving it to other agencies like mental health services.

Mr Hall added that police should have been able to give more information to the media about the suspected attacker in the aftermath of the Southport murders.

“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.”

“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,” he said.