GPs have been attacked by a minister for operating "gentlemen's agreements" whereby they promise not to accept other doctors' patients.
In a BBC News website interview, Ben Bradshaw accused family doctors in some areas of blocking patient choice.
The government's latest review of the NHS in England promises more patient choice in primary care, with incentives for GPs to take on new patients.
Doctors' leaders said Mr Bradshaw's comments were "absolute nonsense".
The attack is threatening to open up a new rift between family doctors and the government.
Mr Bradshaw has already clashed with doctors in recent months over extending opening hours and polyclinics.
But he has now turned up the heat on doctors as the two sides discuss ways of revamping the funding of GP care to encourage them to take on new patients.
For the last few years the government has encouraged patient choice for hospital treatment in England, to foster competition as a way of raising standards of care. Now it wants to do the same with GPs.
At present, GPs are paid between £54 and £125 for each patient depending on how complex their needs are.
But Mr Bradshaw said this should be increased by phasing out the lump sum doctors are given in their contract.
GP pay is essentially divided into two parts.
There is the basic pay, which for most includes a lump sum plus the money for each patient on their list, while the second part is effectively a bonus for offering a range of extra services such as diabetes care and their overall performance as measured against targets.
The lump sum element is given to nine in 10 GPs and is worth £580m - 7.5% of the funds invested in family doctor services.
It varies greatly from doctor to doctor as it was introduced with the new GP contract in 2004 to protect them from losing out under the deal.
Mr Bradshaw said the lump sum "dampened the incentive" to attract new patients and meant some doctors were able to survive with very few patients.
He said government research had found one practice in the south of England with just two patients, but he refused to say exactly where that was. Nor could he say how widespread the issues were.
He added the introduction of choice in GP care could drive up standards in the same way it had for hospital care.
It is absolute nonsense to suggest there are gentleman's agreements - it just doesn't happen
But he warned the current system was working against that as doctors did not want to be seen to be poaching their colleagues' patients.
"There is no doubt there are some areas where gentlemen's agreements operate that mitigate against lists being open to new patients and therefore work against real patient choice."
Mr Bradshaw made the comments ahead of publication of the government's primary and community care strategy on Thursday.
The strategy, which builds on the Darzi review published on Monday, set out a vision for a more personalised GP service.
It called for more use of e-mail and telephone GP consultations - these only happen in a minority of cases currently - and a greater focus on preventing ill-health by creating closer links between practices and a host of community services such as smoking cessation and exercise classes.
The British Medical Association, the doctors' trade union, is already in talks with government negotiators about the funding issue.
While any future changes will affect doctors across the UK, in reality the pressue to compete for patients will only be applied in England as ministers in the other nations do not share such an ideology to the health service.
Laurence Buckman, chairman of the BMA's GPs committee, said he was not opposed to phasing out the lump sum and putting more weight on the size of GP lists.
But he added: "It is absolute nonsense to suggest there are gentlemen's agreements - it just doesn't happen.
"Nor are we going to compete for patients, that is not the way general practice works."
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