By Michael Holden

LONDON (Reuters) - Lawyers for British nurse Lucy Letby on Tuesday launched a new bid to overthrow her conviction for killing seven babies, after a group of international medical experts who reexamined evidence used at her trial concluded none had actually been murdered.

Letby was convicted of killing the newborns and attempting to murder eight more between June 2015 and June 2016 while working in the neonatal unit of the Countess of Chester Hospital in Chester, northern England.

As Britain's worst serial child killer of modern times, she is serving 15 full life terms in prison and was told she would never be released. Letby, 35, has maintained her innocence throughout but has been refused permission to appeal against her convictions.

However, since her trial, medical specialists and other supporters have questioned her guilt, suggesting that expert evidence presented by the prosecution to the jury was flawed.

Her lawyer Mark McDonald said new medical findings from the international experts "demolished" the case against her.

"There is overwhelming evidence that the conviction is unsafe," he said. "If (the experts) are correct, no crime was committed."

He has submitted a preliminary application to the Criminal Cases Review Commission (CCRC), which examines potential miscarriages of justice. It said work was underway to assess the filing.

At her trial, prosecutors said Letby had killed the five baby boys and two baby girls by injecting the infants with insulin or air, or force-feeding them milk.

But University of Toronto Professor Emeritus and neonatal doctor Shoo Lee, who says his research was misused at the trial, told a media conference in London that a group of 14 international experts had examined the medical evidence and concluded it did not show that any babies had been attacked.

Instead there had been medical mistakes and failures at the hospital unit.

"In summary, ladies and gentlemen, we did not find any murders," Lee said. "In all cases, death or injury was due to natural causes or just bad medical care."

A spokesperson for the CCRC said it was not possible for the commission, whose job is not to determine guilt or innocence, to say how long its review would take.

"We are aware that there has been a great deal of speculation and commentary surrounding Lucy Letby’s case, much of it from parties with only a partial view of the evidence," a CCRC spokesperson said.

In the wake of her conviction in 2023, the government set up a public inquiry to examine how the murders went undetected and to review the hospital's response to concerns raised by doctors about Letby before her arrest.

Cheshire Police are also carrying out two further investigations into Letby, and in December said they had questioned her about the deaths of other children.

Police say the guilty verdict against Letby was reached after a trial that had lasted almost a year and following more than a month of jury deliberations.

"The case was subsequently reviewed by the Court of Appeal, where three judges unanimously rejected Letby's claim that the evidence was flawed and dismissed her leave to appeal on all grounds," the force said in a statement.

(Reporting by Michael Holden; Editing by Aidan Lewis)