The Care Quality Commission (CQC) launched its investigation into out-of-hours provider Take Care Now (TCN) after Dr Ubani gave patient David Gray a fatal dose of diamorphine in 2008.
Prior to Mr Gray's death, the firm did not take sufficient action to ensure safe use of diamorphine, the report says.
It found that TCN ignored warnings from a senior clinician that a patient could die from an overdose of the drug.
The findings come as reports suggest Dr Ubani faces a fitness-to-practise hearing with medical authorities in Germany.
The CQC also found that staffing levels at TCN were potentially unsafe and a lack of clinical cover could have 'compromised the care of patients'.
It criticised TCN for failing to learn from serious incidents, and for expanding too rapidly without the clinical governance in place to ensure the quality of its services.
The report concluded: 'TCN was dominated by an agenda of growth and focused on winning contracts, not on providing high quality care.
'TCN did not have effective governance arrangements in place and failed to identify shortcomings and implement changes. In the case of Mr Gray, the failure to identify and mitigate the risks reduced the chance of averting the tragic outcome.'
CQC chairman Dame Jo Williams said TCN 'failed on many fronts'.
'TCN is no longer in operation, but the lessons of its failure must resonate across the health service,' she said.
Chief executive of the GMC Niall Dickson said the report highlighted 'disturbing failures' in out-of-hours care.
'We are working with the DoH to close the current gaps in our regulatory system and protect patients. We are pleased that the government has made this issue a priority.'
TCN has been taken over by Harmoni and is now trading as Suffolk Integrated Healthcare.