A retired GP cleared of murdering three patients with massive drug overdoses is to be investigated over his role in the deaths of another 12 people, police said today.

Dr Howard Martin, 71, was cleared by a jury at Teesside Crown Court yesterday of killing Frank Moss, 59, Stanley Weldon and Harry Gittins, both 74.

Durham Police announced today that coroner Andrew Tweddle is to investigate the deaths of a further 12 patients who were treated by Dr Martin.

Detective Superintendent Harry Stephenson, who led the investigation into Dr Martin, said 12 files will be handed over to the coroner for consideration.

''Ultimately, it will be for him to decide what action to take and what parameters he intends to set for any public hearing,'' he told reporters at a briefing at police HQ in Durham City.

The 12 files consist of three ''in-depth'' inquiries into the deaths of women aged 61, 54 and 78, and ''mini files'' on seven men aged between 58 and 86 and two women in their 60s.

Mr Stephenson said the 12 files were all studied by the Crown Prosecution Service but resulted in no criminal charges because of insufficient evidence.