The BBC's Andrew Tomlinson: "Parents say they never put their signature to anything"

An independent inquiry is already under way into how trials of the ventilator at Stoke-on-Trent's North Staffordshire Hospital were run.

But police have now begun preliminary investigations into the claims that some consent forms, usually signed by parents prior to their babies being treated, did not bear genuine signatures.

Parent Sharon Bradley says she did not sign the form

Gloria Brown, two of whose babies died after spending time in the ventilator, says that a signature on a consent form was not hers.

She said that, in a highly-medicated state following an emergency Caesarian section, she would not have been able to write straight.

She told the BBC: "They fraudulently put the signature there. I did not sign that form."

Two other mothers have made similar allegations.

Categorical denial

Of 122 premature babies treated with the ventilator, 28 died and 15 suffered brain damage. The hospital says these levels are no worse than would be expected on standard ventilator machines.

Parents whose babies died also claimed they were told the ventilators were the latest technology and not that they were being used in a clinical trial.

Professor David Southall ran the trial

The hospital categorically denied this, saying that information was given to parents in line with national standards.

On Wednesday a spokesman said: "The hospital also holds copies of an information sheet, signed by parents, stating that they had read and understood the nature of the study.

"There is no evidence to support the implication that any disability or death resulted as a consequence of the study.

"The study was approved by the local research ethics committee. The information given to parents about the study and the procedures for obtaining their consent were fully in line with national guidelines for research involving children."

Study will be presented to ministers

The ventilator machine was designed to help premature babies breathe in a non-invasive way.

The study, which ran from 1989 to 1993, involved 244 babies - half using the new CNEP ventilator, the remaining 122 receiving conventional treatment.

The continuous negative extrathoracic pressure incubator (CNEP) used suction to help expand the lungs of a premature baby, rather than the standard method, which is to push a tube into the throat.

The independent investigation is checking parents' claims that they were not told their babies were part of a trial, and allegations that the trial continued even though it became apparent that CNEP was not as good as the existing ventilators.

Paediatrician previously in news

It is expected this will be presented to ministers within the next couple of weeks.

The trial was run by Professor David Southall, a well-repected paediatrician, although he would not have been responsible for collecting the consent signatures of patients.

He has previously attracted publicity for a scheme in which parents suspected of harming their babies were invited to spend the night with them in hospital, then secretly filmed.