Starmer defended his government's stance to not divulge many details earlier about the British teenager guilty of the killings by saying he did not want to collapse the case, but admitted there were failings.
Axel Rudakubana, 18, unexpectedly pleaded guilty on Monday to murdering Bebe King, 6, Elsie Dot Stancombe, 7, and Alice Dasilva Aguiar, 9, in the English town of Southport in July, as well as to producing the deadly poison ricin and possessing an al Qaeda training manual.
Axel Rudakubana pleaded guilty and will be sentenced on Thursday. (AAP PHOTOS)
Shortly afterwards, the government said Rudakubana had been referred to Prevent, a counter-radicalisation scheme, three times, but no action had been taken. Police said no one ideology had been uncovered and the case was not treated as terrorism.
It has now launched a public inquiry into the murders.
The attack led to riots around the UK after false reports on social media said the suspect was a radical Islamist migrant, and opponents have accused Starmer and the police of covering up what they knew.
The prime minister said failures would be made public and the attack could show that Britain faces a new type of terrorism threat waged by "loners, misfits, young men in their bedrooms" committing extreme violence.
"I want to be crystal clear in front of the British people today. We will leave no stone unturned ... Nothing will be off the table in this inquiry. It will lead to change," Starmer told a press conference on Tuesday.
The Conservative Party welcomed the inquiry but said "there are many questions that remain unanswered", adding those who failed should be named.
The leader of the increasingly popular right-wing Reform UK party, Brexit campaigner Nigel Farage, has branded handling of the case "the most appalling cover-up I think I've ever seen".
At the swiftly arranged press conference, Starmer was keen to get ahead of any new criticism.
In promising he would call out state institutions that had failed to prevent the killings, he hoped to lift the pressure on his government, which has struggled since winning power in July.
Starmer also faced criticism when he was England's chief prosecutor over his handling of a child sexual abuse scandal.
He said at the time, he had brought the men of mainly Pakistani heritage to justice and vowed, as prime minister, he would change the system after Southport.
"The senseless, barbaric murders of three young girls killed in Southport is a devastating moment in our history," he said. "Southport will be a line in the sand."
The crime shocked Britain and led to a national outpouring of grief for the young victims. (AP PHOTO)
UK Home Secretary Yvette Cooper said a public inquiry would "get to the truth about what happened and what needs to change".
The attack occurred on the first day of summer holidays when the little girls at the Hart Space, a sanctuary hidden behind a row of houses, were in a class to learn yoga and dance to the songs of Taylor Swift.
What was supposed to be a day of joy turned to terror and heartbreak when Rudakubana, armed with a knife, intruded and began stabbing the girls and their teacher in the seaside town of Southport in northwest England.
"This was an unspeakable attack - one which left an enduring mark on our community and the nation for its savagery and senselessness," Deputy Chief Crown Prosecutor Ursula Doyle said.
"A day which should have been one of carefree innocence; of children enjoying a dance workshop and making friendship bracelets, became a scene of the darkest horror as Axel Rudakubana carried out his meticulously planned rampage."
Prosecutors haven't said what they believe led Rudakubana - who was days shy of his 18th birthday - to commit the atrocities but Doyle said that it was clear he had a "a sickening and sustained interest in death and violence".
He pleaded guilty to three counts of murder, 10 counts of attempted murder, possession of a knife and additional charges related to possessing the poison ricin and for having an al-Qaeda manual.
Rudakubana faces life imprisonment when sentenced on Thursday, Justice Julian Goose said.
He pleaded guilty to murdering Alice Da Silva Aguiar, 9, Elsie Dot Stancombe, 7, and Bebe King, 6.
with agencies
