The NHS could face high bills if hospitals and surgeries are required to install biometric scanners to verify the identity of patients, the British Medical Association warned.
Hospitals would face further costs if they had to employ people to recover treatment costs from patients who turn out not to be eligible, said Vivienne Nathanson, the BMA's director of professional activities.
John Chisholm, chairman of the BMA's general practitioners committee, said doctors were broadly in favour of ensuring that treatment was limited to those entitled to it. But they were concerned that ID checks could use up valuable patient time.
"Our preference would be to establish identity at the time of registration, with particular allowance made for the frail and disabled. Doctors do not want to see ID cards every time a patient is seen," he said.
Both groups said there were problems with linking ID cards to patient health records.
There was a risk that people would be less willing to seek treatment if they felt their health records could be deliberately or accidentally disclosed to third parties, said Chisholm.
Nathanson said that doctors would be reluctant to trust medical information, such as details of allergies stored on ID cards, as there were no guarantees that the information was up to date.