Lucy Letby, the British nurse convicted last week of killing seven newborns and trying to kill six others, was sentenced on Monday to life in prison without parole, the culmination of a yearslong case that has horrified Britain and led to questions over the management culture that allowed her crimes to continue for so long.

Judge James Goss handed Ms. Letby a “whole life order,” meaning she will spend the rest of her life in prison, a sentence reserved for the country’s worst offenses. She is only the fourth woman to have ever been handed the sentence.

The verdicts reached last week made Ms. Letby the most prolific serial killer of children in modern British history.

Judge Goss told the courtroom that Ms. Letby “acted completely contrary to the normal human instincts of nurturing and caring for babies” and that her actions caused a majority of her victims to suffer “acute pain.”

“There was premeditation, calculation and cunning in your actions,” the judge said, later describing “a deep malevolence bordering on sadism” in Ms. Letby’s crimes.

The murders and attempted killings took place between June 2015 and June 2016, when Ms. Letby was a nurse in the neonatal ward of the Countess of Chester hospital in northwestern England, tasked with caring for premature and vulnerable babies. She refused to appear in court during her sentencing on Monday, but the court heard heart-wrenching testimony from the parents of babies who were killed.

After the first guilty verdicts were announced, Ms. Letby told her legal team that she would refuse to attend any further court proceedings, and she did not leave her jail cell on Monday, prompting politicians to debate ways to force convicted criminals to hear their sentencing.

The case has also led to calls for an investigation into the possible systemic failures that led to Ms. Letby’s continued employment as a nurse in the hospital, even after doctors raised concerns about her work, details of which were revealed during her trial and in its immediate aftermath.

The mother of a baby boy killed by Ms. Letby addressed the absent former nurse in court on Monday, saying, “There is no sentence that will ever compare to the excruciating agony that we have suffered as a consequence of your actions,” according to the BBC.

The mother detailed how she had a memory box of mementos of her son and had treasured his hand and foot prints. But she said she now felt conflicted because Ms. Letby was the nurse who made the prints when he was born.

The father of triplets — two of whom Ms. Letby was convicted of killing — said in a prerecorded video statement that “everyday life was difficult, just getting up and living was a struggle,” after the death of his children.

“Lucy Letby has destroyed our lives,” he said, adding, “Even after the trial has ended, it will continue to haunt us and will always have an impact on our lives.”

The lawyer for the prosecution, Philip Astbury, read out a statement from the parents of twins who were attacked in June 2015. One survived, the other died, and Ms. Letby was convicted of their murder and attempted murder.

The statement from the twins’ parents described their trauma after the one child died, and said there had always been a family member present with their surviving daughter, but that Ms. Letby had been waiting for the relatives to leave so she could “attack.”

“You thought it was your right to play God with our children’s lives,” the statement said. “You thought you could enter our lives and turn it upside down, but you will never win. We hope you live a very long life and spend every day suffering for what you’ve done.”

While some of the parents delivered their statements through lawyers, others spoke directly to the courtroom, through tears. One mother of two victims said that Ms. Letby’s absence from court showed the “disrespect” she had for the court and the families.

“We have attended court day in and day out, yet she decides she has had enough, and stays in her cell, just one final act of wickedness from a coward,” the mother said. The identities of the babies in the case and their families were protected throughout the trial, with the infants each referred to by a single letter throughout the proceedings.

Prime Minister Rishi Sunak, speaking on Monday morning, said that like everyone reading about the case, he had been shocked by the harrowing details.

“I think it is cowardly that people that commit such horrendous crimes do not face their victims and hear firsthand the impact that their crimes have had on them and their families and loved ones,” he said, adding that the government was looking at changing the law to make sure convicted criminals attended their sentencings.

Over the course of the 10-month trial at Manchester Crown Court, which began in October, jurors heard that Ms. Letby had harmed babies by overfeeding them with milk, injecting them with air and insulin and inflicting “impact-type” trauma.

“In her hands, innocuous substances like air, milk, fluids — or medication like insulin — would become lethal,” Pascale Jones, who worked on the prosecution team for the case, said after the verdicts were delivered. “She perverted her learning and weaponized her craft to inflict harm, grief and death.”

Ms. Letby, 33, maintained her innocence throughout the trial, where she faced 22 counts related to the killing and harming of babies. In addition to the murder convictions, Ms. Letby was found guilty of seven counts of attempted murder related to six newborns, meaning she tried to kill one of them twice, prosecutors said. The jury did not reach verdicts on six counts of attempted murder and Ms. Letby was found not guilty on two counts of attempted murder.

Medical records, text and social media messages, staffing schedules and handwritten notes and diaries were used to help convict Ms. Letby, prosecutors said.

The government has ordered an independent inquiry into how Ms. Letby managed to evade detection for years after the British news media reported that hospital managers had ignored repeated warnings about her conduct.

A BBC investigation revealed that doctors at the hospital had first raised concerns about Ms. Letby in 2015, but alleged that the concerns were dismissed amid a culture within the hospital that discouraged whistle-blowers from raising alarms about potential failings. The Countess of Chester Hospital has new management and its current medical director, Dr. Nigel Scawn, said the entire organization was “deeply saddened and appalled” by Ms. Letby’s crimes.

“Our staff are devastated by what happened and we are committed to ensuring lessons continue to be learnt,” he said in a statement.

The Cheshire Constabulary are still investigating whether other babies who had contact with Ms. Letby had experienced unexpected health issues, and the authorities are calling on anyone with additional information to contact them. The attacks in the current case only relate to deaths within one year.

Kaly Soto contributed reporting.