German GP Daniel Ubani - on his first NHS shift - gave patient David Gray, 70, an overdose of diamorphine.
He was working for Take Care Now, a company that provided care in Cambridgeshire.
Coroner William Morris said the doctor had been "incompetent" and called for a shake-up of the out-of-hours care system to ensure patient safety.
... Dr Ubani had made "a gross error" and was "not of an acceptable standard" in carrying out his duties in Cambridgeshire in February 2008.
But he also criticised the "insufficient induction" he was given by Take Care Now as well as the NHS system which meant his skills were not properly assessed. The company's contract has now been terminated.
Mr Morris called for a review of European regulations which allow free movement of doctors, a national database of overseas doctors applying to work in out-of-hours services in the NHS, and more consistent standards in monitoring by local health chiefs working for primary care trusts.
A second patient Dr Ubani had treated - Iris Edwards, 86 - was ruled by the inquest to have died of natural causes.
Dr Ubani had had an application to work in the UK rejected by the NHS in Leeds because of inadequate English.
But he later got approval in Cornwall after they did not test his language skills.
Mr Morris also said there should be better assessment by the NHS of overseas doctors' ability to work in the UK.
... Mr Gray was suffering from renal colic when he was given 10 times the normal amount of the painkiller diamorphine by Dr Ubani at his home in Manea, Cambridgeshire on February 16 2008, and was pronounced dead fours hour later, the hearing was told.
In April 2009, the German authorities gave Dr Ubani a nine-month prison sentence, suspended for two years, and he was ordered to pay a £4,500 fine for causing Mr Gray's death by negligence.
But he continues to practise as a cosmetic surgeon in the west German town of Witten.