Britain's most prolific serial killer Harold Shipman is unlikely to face further prosecution despite the publication of a report yesterday suggesting he may have killed more than 300 patients during a 24-year career of murder.

The enormity of the crimes were revealed in an exhaustive study of his patients' records and medical certificates. This was conducted by Prof Richard Baker, professor of quality in healthcare at Leicester University and commissioned by the health secretary, Mr Alun Milburn.

Prof Baker's 145-page report found an "excess" of 297 death certificates - including the 15 patients he was convicted of killing last year - among Shipman's patients compared with the death rates of other GPs in the same area during his career in West Yorkshire and Greater Manchester.

The report indicated that, in total, 345 deaths among his patients needed further explanation, but a realistic figure for the number of suspicious deaths was likely to be around 236.

However, Prof Baker said it would be impossible to pinpoint the exact number of deaths Shipman was responsible for unless he admitted the extent of his crimes.

"It is very hard to contemplate these findings without being concerned for the hurt and distress caused to people," Prof Baker said.

The former GP was convicted in January last year of killing 15 elderly women patients in the Hyde area of Greater Manchester by administering lethal doses of diamorphine.

One of Shipman's confirmed victims was Ms Maureen Ward (57), originally from Derry, who died in Manchester in 1998. He told her family she was suffering from a brain tumour but during his trial it emerged he had falsified her medical records. Ms Ward's mother, Muriel Ward (87), also from Derry, died when she was a patient of Shipman's in 1995 and is included on a list of probable murder victims.

After he was given 15 life sentences, police said they had enough information to charge Shipman with the murder of another 23 patients, but the Crown Prosecution Service (CPS) ruled out further legal action because it would be impossible for Shipman to get a fair trial.

The Director of Public Prosecutions said it was unlikely Shipman would face further prosecution.

And the head of the CPS, Mr David Calvert-Smith, QC, said he shared that belief yesterday when he declared that despite the publication of Prof Baker's report he did not believe a jury could try Shipman "fairly and objectively".

A public inquiry into Harold Shipman's crimes is expected to open next month, headed by the High Court judge, Dame Janet Smith.

Shipman issued a total of 521 death certificates at his Hyde practice, where he worked from 1977 to 1998, compared with the highest number issued by another GP in the area during the same period of 210.

The highest death rates during his career were among women aged between 64 and 75 and among older men and deaths occurred mainly between 12.00 p.m. and 3 p.m. when patients were likely to be alone in their homes.