The Health and Safety Executive will continue its own investigation
Manslaughter charges will not be brought in connection with the death of a man overcome by fumes.
Matthew Pitt, 29, of Sturminster Newton, died in hospital after the incident at Lowbrook Dairy Farm at Belchalwell, in Dorset, on 24 June.
The agricultural contractor was using a piece of sewage treatment equipment.
Police said it made the decision after carrying out an investigation with the Health and Safety Executive, which is still examining the incident.
A spokesman for Dorset Police said: "Dorset Police and the Health and Safety Executive (HSE) commenced a joint investigation.
"On Friday, 17 July, following a meeting with the Crown Prosecution [Service] - overseeing the offence - a decision was reached that there was insufficient evidence to support a prosecution for manslaughter or corporate manslaughter.
"In accordance with the work-related deaths protocol and by agreement with Dorset Police and the HSE, primacy of the investigation will now pass from the police to the HSE to consider whether any health and safety offences were committed as a result of the tragic incident."
The inquest into Mr Pitt's death was opened and adjourned in Blandford on 29 June.
A 50-year-old Blandford man was also treated in hospital for breathing difficulties but was later discharged following the incident.