Veteran MP Sir David Davis has a history of championing successful miscarriage of justice cases.
Most recently, he helped Mike Lynch, the tech entrepreneur who successfully fought a 12-year legal battle in the USA, but died after a freak storm engulfed his yacht earlier this month.
Sir David said he had started off by thinking that Letby was guilty.
His doubts began in May after he raised a question in the House of Commons on why a critical piece in an American magazine was not allowed to be published here.
This was before the second trial and British contempt laws do not allow publication of anything which could influence the jury.
"It was only the fact that I got authoritative calls from people who really know about statistics, about medicine, about science, about law, and I’d never had anything like this happen before," he said
"I started to think – it’s a terrible crime, but if they’ve got it wrong, it’s a terrible miscarriage of justice."
Sir David said he believed other possibilities for the deaths could have included a lack of staffing and training on the unit and an infectious outbreak, possibly linked to the faulty drainage discussed in the trial.
"All of us find it easier to believe that a villain has killed people rather than a system or a random act," he said.
He is now reading through thousands of documents detailing the trial before making a decision on whether to take the case up and press for the Criminal Cases Review Commission to get involved.
Sir David said he already believes that the trial was flawed but by itself that does not mean Letby is not guilty.
He added that he will not take it further unless he comes to the conclusion that she was probably innocent.