Doctors' leaders are concerned over a scheme where GPs are paid when patients have surgery at an independent centre.
GPs are being paid to cover the costs of tests
Ashton, Leigh and Wigan Primary Care Trust is paying GPs £30 each time a patient chooses to be treated at the Greater Manchester Surgical Centre.
It says the payment covers tests needed to determine if they are suitable for treatment at the Netcare centre.
But the British Medical Association warned the payments could be seen as financial inducements.
The local NHS has said it is paying GPs for checks such as blood tests and admin checks which, if patients go to other centres, would be carried out at out-patient appointments.
The scheme has been running since January.
Ashton, Wigan and Leigh PCT signed a contract with the surgical centre for it to provide operations to NHS patients.
The PCT denies offering financial inducements.
In a statement, it added: "Our early experience of the scheme is positive with more that 90% of the patients referred by GP practices meeting the inclusion criteria and hence being able to move quickly through for their surgical intervention.
"This early experience suggests the scheme offers excellent value for money - of course we will continue to evaluate rigorously.
"Patient satisfaction of the centre is very high and 390 patients have so far been treated."
It added 14 practices had made direct referrals as part of the scheme.
But Dr Hamish Meldrum, the head of the British Medical Association's GPs' committee, said GPs would always have to organise tests, wherever a patient was being treated.
He said the BMA was telling local doctors it would be "unwise to accept payments that could be perceived as financial inducements".
He added: "This seems to be a disguised financial incentive to refer to a particular centre.
"We would suggest the PCT withdraw the payment scheme."
A spokesman for Netcare said the payments were made to GPs after patients had chosen where to have their operation and required certain tests and checks - and therefore could not be financial inducements.
She said patients were first seen at their local hospital and told what the wait for their operation would be there.
A patient choice co-ordinator from the PCT would then call and discuss the options at other centres.
If they chose Netcare, the organisation would require certain information to ensure the patient is suitable for the surgery, such as blood pressure checks and information about any underlying conditions.
She added: "The Greater Manchester surgical centre does not receive direct referrals from GPs.
"Netcare is unable to comment on arrangements between a PCT and GPs in its local health economy."