Around one in 10 GP practices in England will be privately run by 2014, medics have predicted.
According to a study by GP newspaper, 4% of surgeries in England are now run by private businesses, and this looks likely to increase.
Dr David Jenner from NHS Alliance predicted that one in 10 will be privately run in three years, more than double the current proportion.
This is partly because the local health authorities called primary care trusts (PCTs) are set to be abolished, leaving a gap in terms of who will run around 80 practices that are currently PCT-controlled.
Surgeries run by one GP are particularly vulnerable to shutting down in the face of radical changes to the NHS, the newspaper said, and the local service then being taken over by a private provider.
It found that there were 191 privately run practices across 92 PCTs in England, which extrapolates to 315 out of 8,300 surgeries across the country.
Chairwoman of the Royal College of General Practitioners Dr Clare Gerada said: "It is inevitable that the number of privately run GP practices will increase. What's going to stop them?"
General Practitioners Committee deputy chairman Dr Richard Vautrey warned that there was a danger of the quality of care dropping if large businesses were involved.
A Department of Health spokesman said: "Patients will never have to pay for NHS care and we will never privatise the NHS.
"The amount of independent sector provision in the NHS will be determined by whether or not patients want it, not by Government."