Investigations into the deaths of a further 12 patients of retired family doctor Howard Martin are to continue, police announced yesterday.

Durham Police have prepared files on 12 patients - understood to include a father and son - where morphine may have played a significant factor in their deaths.

The news came less than 24 hours after a jury at Teesside Crown Court unanimously found the 71-year-old not guilty of murdering three seriously-ill patients with huge overdoses of morphine.

The three - Frank Moss, 59, Stanley Weldon and Harry Gittins, both 74 - were all patients of Dr Martin when he was a partner at the Jubilee Medical Group, which has surgeries in Newton Aycliffe, Shildon and Eldon, in County Durham.

Detective Superintendent Harry Stephenson, who led Durham Police's largest murder investigation, said: "The deaths of a number of other patients of Dr Martin, where morphine could have made a significant contribution, have been subject of detailed scrutiny by both the police and the Crown Prosecution Service.

"We now intend to hand the 12 files we currently hold to the Durham Coroner, Andrew Tweddle. Ultimately, it will be for him to decide what action to take and what parameters he intends to set for any public hearing."

The files consist of three in-depth inquiries into the deaths of women aged 54, 61 and 78, and "mini-files" on seven men, aged between 58 and 86, and two women in their 60s.

They are thought to include a father and son from the Newton Aycliffe area.

Billy Sefton died in 1983 after contracting cancer.

His father, George, died in 1992 after a long period of ill health. His family are reported to have confirmed both were patients of Dr Martin.

Det Supt Stephenson said 28 complaints were referred to the police, either by relatives or the local health authority.

He revealed police were first called to examine Dr Martin's conduct in December 2000 after the death of a 61-year-old woman.

Macmillan Nurses had questioned his management of the woman's care but, using evidence available at the time, they concluded her treatment had been acceptable.

A further 12 complaints from patient's families, relating to deaths and prescribing methods, were considered by police but set aside because of a lack of forensic evidence, medical records and witness statements.

Last night, Dr Martin, who now lives in North Wales, declined to comment.

But solicitor Sara Mason, of the Medical Defence Union, said it was clear "that the Crown Prosecution Service has no intention of bringing any further charges against Dr Martin in relation to the death of any former patient".

The bodies of Mr Gittins, of Newton Aycliffe, Mr Moss, of Eldon, near Bishop Auckland and Mr Weldon, of Coundon Grange, were exhumed as part of the original investigation, along with that of a fourth man, William Kerr, 84, of Newton Aycliffe.

Mr Tweddle has opened an inquest into the death of Mr Kerr, in March, 2003, which is yet to be concluded.

Last night, Mr Gittins' daughter, Gillian Coates, said: "I feel for the families concerned but if they believe in it and feel that they need some answers to questions then they have no choice but to go through with it."

Speaking from her home in Wales, Dr Martin's step-daughter, Rosie Williams, said Durham Police had been over-zealous.

She said: "He (Dr Martin) goes out of his way to help people and he would never do anything that was not in the best interests of a person."

Det Supt Stephenson said: "From day one, this has not been a personal vendetta. We were charged with investigating the fact put to us. There is a legal and moral duty to do that."

Alongside the police investigation, Sedgefield Primary Care Trust is also conducting a review of Dr Martin's patients' medical records which it expects to complete in the New Year