An 18-year-old British man Monday admitted to murdering three girls and attempting to kill 10 other people during a brutal knife attack at a dance class last July in Southport, England.

The defendant, Axel Rudakubana, unexpectedly pleaded guilty to all charges against him on the first day of his trial at Liverpool Crown Court.

Mr. Rudakubana admitted to killing Bebe King, 6, Elsie Dot Stancombe, 7, and Alice da Silva Aguiar, 9, who were taking part in a Taylor Swift-themed dance and bracelet-making class on July 29 during their summer break from school, and to the attempted murder of 10 people, including eight children and two adults.

After the stabbings, the police searched Mr. Rudakubana’s house and said they found ricin, a lethal toxin, and a PDF file titled “Military Studies in the Jihad Against the Tyrants: The Al Qaeda Training Manual.” Mr. Rudakubana was subsequently charged with production of a biological toxin and with “possessing information of a kind likely to be useful to a person committing or preparing an act of terrorism.”

On Monday, Mr. Rudakubana, who had initially pleaded not guilty, appeared in court, sitting at the back of the room in the defense dock and wearing a gray sweatsuit, though he repeatedly refused requests from the court to identify himself or to speak at a volume that the judge could hear.

The BBC reported that Mr. Rudakubana, whose mouth was covered with a surgical mask, uttered a mumbled “guilty” as each count in the indictment against him was read out.

His defense lawyer, Stanley Reiz, confirmed that the defendant understood the proceedings and had pleaded guilty to all of the charges against him. His admission of guilt came as the court was preparing to hear days of evidence about the harrowing attack last summer.

Now, the case will move swiftly to sentencing, according to Judge Julian Goose, who was presiding over the case and ordered that Mr. Rudakubana be sentenced on Thursday.

“You will understand it is inevitable the sentence to be imposed upon you will mean a life sentence equivalent,” he told Mr. Rudakubana after the guilty pleas.

Judge Goose said he “extends his apologies” to the families of the victims, as none were in court on Monday, because the initial evidence in the trial was not expected to be heard until later in the week. But he confirmed that those families would be in the courtroom on Thursday.

Ursula Doyle, the deputy chief crown prosecutor for the Mersey-Cheshire region, said in a statement that the “unspeakable attack” had “left an enduring mark on our community and the nation for its savagery and senselessness.”

“It is clear that this was a young man with a sickening and sustained interest in death and violence,” she said, adding that Mr. Rudakubana had shown no sign of remorse. “The prosecution was determined to prove his guilt and I am deeply grateful that today’s plea has spared the families at the heart of this case the pain of having to relive their ordeal through a trial.”

The authorities have declined to declare the stabbing attack an act of terrorism. Serena Kennedy, the chief constable of the Merseyside Police, which oversees policing in the region, said in a statement in October that “for a matter to be declared a terrorist incident, motivation would need to be established.” She added, “We would strongly advise caution against anyone speculating as to motivation in this case.”

Mr. Rudakubana was born in Cardiff, Wales, to parents who were originally from Rwanda. He was living in Banks, a village north of Southport, at the time of the attack.

After the Southport attack, Britain was convulsed by a series of violent riots, as disinformation about the attacker’s identity swirled on social media and messaging apps. False claims that the suspect was an undocumented immigrant or newly arrived asylum seeker were amplified by anti-immigration activists and members of the far right.

Several people, including a neo-Nazi, were later found to have helped coordinate outbreaks of unrest, which included attacks on mosques and hotels where asylum seekers were staying, and which led to dozens of police officers being injured.

Mr. Rudakubana was 17 at the time of the attack and, under English court rules, he would usually have retained his anonymity until he turned 18. But a few days after the attack, a judge took the unusual step of releasing his name in an attempt to combat the spread of misinformation.

Since the riots last summer, hundreds of people have been charged for their involvement in the violent disorder across the country, and dozens of people have been sentenced to prison time.

Yvette Cooper, Britain’s home secretary, said in a statement on Monday morning that the trial would be a “deeply traumatic and distressing time for the families of Alice, Bebe and Elsie, the survivors, and the people of Southport and the whole country.”

“The most important thing for all of us is to ensure that the legal process can take its course, to respect the difficult job the court has to do so there is a fair trial and justice can be done,” she added.