David Gray, 70, died after being injected with 100 milligrammes of diamorphine - 10 times the recommended daily dose - by Dr. Daniel Ubani in February 2008, a 10-day inquest heard.
The German doctor was on his first out-of-hours shift as an overseas "locum" doctor in Britain - typically used to provide care when a regular family doctor is unavailable, for example at night or on the weekend.
Coroner William Morris described Ubani as "incompetent and not of an acceptable standard," and criticised the out-of-hours arrangements, saying: "Weaknesses remain in the system."
Gray's family called for the German doctor to face trial in Britain. Ubani cannot be extradited to Britain because he has already faced trial in Germany, where he was given a suspended prison sentence, British media reported.
The Briton's son Stuart, who is also a doctor, said his father died because of Ubani's actions and due to "serious failings" in the Cambridgeshire Primary Care Trust and Take Care Now, the local health authority and medical provider.
"We want to see him tried under UK law for his death but we also want safeguards put in place nationwide to prevent this happening again," he added.
The head of NHS Cambridgeshire, the National Health Service authority in England where the incident occurred, admitted failings.
"We accept that the systems failed, in that someone with Dr. Ubani's qualifications and experience should not have been put in a position where he was able to make this type of mistake," said Dr. Paul Zollinger-Read.
Out-of-hours health care in Britain has repeatedly come under the spotlight in recent years, following reforms of the system and a number of incidents involving uncoordinated and substandard medical care.