The crime shocked Britain and misinformation about the attacker sparked anti-immigrant violence across the country.
The government announced it would hold an independent public inquiry into the attack, which was carried out by a UK-born teenager whose fascination with violence saw him reported to authorities years before the crime.
Axel Rudakubana, 18, entered the surprise guilty pleas as jury selection had been expected to begin at the start of his trial in Liverpool Crown Court.
Axel Rudakubana had "a sickening and sustained interest in death and violence". (AAP PHOTOS)
The July 29 stabbings led to a week of widespread rioting across parts of England and Northern Ireland after the suspect was falsely identified as an asylum-seeker who had recently arrived in Britain by boat. He was born in Wales to Rwandan parents.
UK Prime Minister Keir Starmer welcomed Rudakubana's conviction, but said it was "a moment of trauma for the nation".
"There are grave questions to answer as to how the state failed in its ultimate duty to protect these young girls," he said.
"Britain will rightly demand answers, and we will leave no stone unturned in that pursuit."
UK Home Secretary Yvette Cooper said that a public inquiry would "get to the truth about what happened and what needs to change".
She disclosed that Rudakubana had been referred to the government's anti-extremism program, Prevent, when he was 13 and 14, and "was in contact with a range of different state agencies throughout his teenage years" — all of whom failed to spot the danger he posed.
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.
The crime shocked Britain and led to a national outpouring of grief for the young victims. (AP PHOTO)
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.
Eight other girls, ranging in age from seven to 13, were wounded, along with instructor Leanne Lucas and John Hayes, who worked in a business next door and intervened. Fifteen other girls, as young as five, were at the class but uninjured.
Police said the stabbings weren't classified as acts of terrorism because the motive wasn't known.
Matt Jukes, head of counterterrorism policing, said that an extensive investigation found Rudakubana had "a wide interest in conflict, violence, genocide, and terrorism" but investigators were unable to figure out what drove him to kill.
Several months after his arrest at the scene of the crime, Rudakubana was charged with additional counts for production of a biological toxin, ricin and possession of information likely to be useful to a person committing or preparing to commit an act of terrorism for having the manual in a document on his computer.
