Private healthcare companies supply "consistently worse" quality of care than GP services provided by the NHS in England, research has found.
A study compared alternative healthcare providers - which include voluntary organisations as well as private sector companies - with NHS primary care services.
Researchers at Imperial College London found alternative providers performed worse than traditional providers on 15 of 17 performance indicators after adjusting for practice and population characteristics.
These included measures such as how easily patients get appointments, how well they manage patients' blood pressure, and cervical screening coverage.
Alternative providers also had worse results for patients' diabetes control, higher hospital admission rates for chronic conditions, and overall patient satisfaction.
The only two areas they performed better in were access to low cost statins and satisfaction with opening hours.
In total, 4.1% (347 out of 8,300) of general practices in England were run by alternative contract providers - which have been contracted to offer primary care in the NHS since 2004 under reforms designed to increase competition.
The study, which is published in the Journal of the Royal Society of Medicine, acknowledged there are a variety of different types of non-NHS providers, ranging from those run by entrepreneurial GP groups to multinational corporations.
It concluded: "Practices run by alternative providers supplied consistently worse quality of care than traditional practices - across a broad range of indicators.
"While some of this difference may be due to the different populations they serve, our models - which adjust for population and practice features and for underlying trends - found that Alternative Provider of Medical Services practices continued to do worse than would be expected compared to the national sample of general practices.
"These findings suggest that allowing new alternative providers into the primary care market in England has not led to better care for patients - and may have even resulted in worse care."
It added that "some of the patterns of poor performance are concerning and merit further examination".
Researchers acknowledged that some practices that were put out to tender may have been more likely to have been practices with a history of poor performance, while others were newly set up.
Alternative providers also tended to be smaller and serve younger, more diverse and more deprived populations than traditional GP surgeries.
Lead author Dr Christopher Millett, of Imperial College London's School of Public Health, said: "This study provides data to inform the debate about the growing role of the private sector in the NHS.
"New providers were allowed into the primary care market to stimulate competition, but our findings suggest that their introduction has not led to improvements in quality and may have resulted in worse care.
"The lesson is that increasing diversity does not necessarily lead to better quality. Regulators should ensure that new providers of NHS services are performing to adequate standards and at least as well as traditional providers."
He added that although alternative providers have not been widely contracted to delivery primary care services, private companies have secured a third of contracts to deliver NHS clinical services since the Health and Social Care Act was enacted in 2013.