A new criminal offence is to be introduced to “close the gap” between lone, violence-obsessed individuals with no particular ideology and terrorism suspects, the home secretary has said.

Yvette Cooper said an offence that would give police the power to apprehend the former long before they acted was needed in the wake of the Southport attack last year. Terrorism suspects can be jailed for life even if their plans are not fully formed.

Axel Rudakubana, then 17, stabbed three young girls to death at a summer holiday dance class in July last year and attempted to murder eight other children and two adults who tried to protect them.

Cooper told BBC Radio 4’s State of Terror series: “There is a gap in the law around the planning of mass attacks that can be just as serious [as terrorism] in their implications for communities, their impact, the devastation that they can cause and the seriousness of the crime.

“We will tighten legislation so that that is taken as seriously as terrorism.”

She said police would get the power to prevent such individuals who did not have a clear ideology, in the same way they can with terrorism suspects.

Under section 5 of the Terrorism Act, a person who engages in any conduct in preparation of a terrorist act is guilty of the offence, with a maximum sentence of life imprisonment.

At the bottom end of the scale, the offence includes engaging in very limited preparation for terrorist activity but there must be a link to an ideological cause.

Cooper said: “We’ve seen cases of growing numbers of teenagers potentially radicalising themselves online and seeing all kinds of extremist material online in their bedrooms.

“They’re seeing a really distorted and warped online world.

“We have to make sure that that the systems can respond while not taking our eye off the ball of the more longstanding ideological threats.”

The Prevent anti-terrorism scheme declined to take on Rudakubana’s case three times, after teachers raised concerns about him from 2019 to 2021, three years before he committed the atrocity.

His first referral was from his teachers after he admitted bringing a knife into school on 10 occasions to “stab someone”, researched massacres of children at US schools and made “graphic” comments about violence. He later said “the [2017] terrorist attack on the MEN [Manchester Arena] was a good thing”.

While there was concern about his interest in violence, Prevent concluded there was no sign it was driven by a terrorist ideology.

A public inquiry into the Southport killings began this month. Its chair, Sir Adrian Fulford, said it would examine the “wholesale failure” of institutions to prevent “one of the most egregious crimes in our country’s history”.

He said he would consider whether courts should be given powers to impose restrictions on individuals known to pose a risk but when there was insufficient evidence to justify an arrest.

Earlier this year, David Gauke’s independent sentencing review said the introduction of new offences and longer sentences was one of the factors behind the prison overcrowding crisis.