The defense team for Lucy Letby, a British nurse found guilty of murdering babies in her care, said it will seek permission from the Court of Appeal to review all of her convictions, saying a lead prosecution expert was not reliable because he had changed his mind about the causes of death of three babies.

The development shines a critical spotlight onto Ms. Letby’s convictions for the deaths of seven babies at a hospital in northern England, in a case that shocked Britain but has increasingly been viewed by some experts as a possible miscarriage of justice.

Dr. Dewi Evans, a retired pediatrician, was the prosecution’s lead expert witness and had testified that air had been injected down the nasal gastric tube of three babies in Ms. Letby’s care. Dr. Evans initially determined that to be their cause of death.

“Remarkably, Dr. Evans has now changed his mind on the cause of death of three of the babies,” Mark McDonald, a defense attorney for Ms. Letby said during a news briefing in London on Monday.

He said that Dr. Evans had given a new report to the police a few months ago, in which he said that he had revised his opinion on the death of one of the babies, known as Baby C. Dr. Evans has also given public statements since the trial, in which he offered differing accounts of the deaths of two other babies.

“Despite numerous requests, the prosecution have yet to give this report to the defense,” Mr. McDonald said, citing the filing to the police. The defense plans to argue to the Court of Appeal that Dr. Evans was “not a reliable expert” and, given that he was the prosecution’s lead expert in the cause of death for all of the babies, that all of Ms. Letby’s convictions were therefore in question.