The warning came after a GP investigation found that 4% of GP practices in England were run by a private provider.
GP leaders said private companies would take over practices currently run by PCTs and step in where singlehanders were forced out by cuts to pensions and NHS reforms.
Data obtained under the Freedom of Information Act showed 67 out of 92 PCTs that responded had at least one practice run by a private company. Two PCTs listed nine such practices in their area.
The investigation found 191 privately run GP practices across the 92 PCTs. This equates to 315 out of 8,300 practices across England's 152 PCTs.
Data from some PCTs showed an increase in the number of practices run by private companies, with 11 moving to private control in 2011/12.
But GP leaders forecast further increases in the near future. NHS Alliance GMS/PMS lead Dr David Jenner predicted that 10% of practices will be run by a private company three years from now - more than double the current proportion.
He said the main drivers for the increase would be private companies capturing PCT-run practices, as PCTs wind down their functions. Around 80 practices are currently run in-house on PCTMS contracts.
He added: 'I think the pension changes and the NHS reforms will force GPs to run to the exit, particularly many older, singlehanded GPs.
'Unless another practice merges with them before they retire, the practice vacancy would be tendered. Clearly there is an opportunity for private providers who are looking to gain market share.'
RCGP chairwoman Dr Clare Gerada said: 'It is inevitable that the number of privately run GP practices will increase. What's going to stop them?'
She said it was 'reasonable' to expect 10% of practices would be run by private companies by 2014.
Dr Jenner said practices should consider merging or federating to prevent single-handed practices being taken over by private companies.
'If GPs are interested in expanding their own practices or keeping private providers at bay they can form partnerships with the smaller ones before any retirements at singlehanded practices.'
GPC deputy chairman Dr Richard Vautrey said it was also up to retiring GPs to think through the future options for their practices. 'What happens to practices is largely in the hands of GPs,' he said. 'In these situations I would hope GPs see the future of the profession is important enough for them to ensure a proper succession.'
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Scotland GPC Scotland chairman Dr Dean Marshall: 'Private providers should be excluded. To have a GP contract in Scotland you have to work in that practice each week. The people who work in the practice have a commitment to that community, which is not the case if the contractor is a private company.'
Wales GPC Wales chairman Dr David Bailey: 'In Wales, the aim is not to have private companies in the NHS. I am not expecting an expansion of private provision. The only APMS contracts in Wales are in out-of-hours.'
Northern Ireland GPC Northern Ireland chairman Dr Brian Dunn: 'There are no private-run practices in Northern Ireland that I am aware of. Practices are all GMS, still like traditional general practice before the 2004 contract. We have advocated federation to prevent private companies coming in.'